This question was asked by my three year old niece and I couldn't think of a good answer.
That's a nice question, but I don't think things can "have" front and back. A chair is just an object with its geometry and configuration. Front and back are concepts defined by us for matters of pure perspective. They are not some kind of property of the chair.
In the course of our existence on earth we found it necessary to distinguish between some visually different parts of objects as we found it necessary to develop numeral systems for counting, but that doesn't mean numbers represent something outside of pieces of information inside our brains or that front and back are real properties of things and not some neurological convention. We made it up.
Anyway, I think this question is harder to answer than it appears, because we would have to talk about properties first and Russell has already shown us that properties are not easy business.
Every surface has a "front and back" locally. One way to answer the question "why?" is to prove that a plane in R^3 separates R^3 into two components (compare with the situation of a line in R^3).
More interesting is the question of whether surfaces have a global front and back. Non orientable surfaces like the Möbius strip, for instance, have only one side (this would be great to show your niece). Having a "front and back" means that the normal bundle of the surface is disconnected. A normal bundle (of a hyper surface) is a device for keeping track of local choices of "front and back" on a surface. In general, a surface (or hyper surface) lying in R^3 (or any orientable manifold) is orientable (meaning it admits a global choice of "left/right") if and only if its normal bundle is disconnected. The disconnectedness of the normal bundle means that a person living on the surface can make a global choice of "front back" at every point in the surface because this choice just amounts to choosing one of the components of the normal bundle.
I find it fun to think about what it would be like to inhabit a non-orientable three dimensional universe. In such a space, you could take a trip and come back as your mirror-self. If you took a piece of black licorice with you (which contains caroway), it would come back smelling like lavender, since the two molecules are chiral pairs.
Physical objects have some universal properties - for example, symmetries and asymmetries.
An object such as a chair is the aggregate of various symmetries and asymmetries. The terms "front" and "back" are names we use to identify a particular type of asymmetry.
In the case of a chair, the front has a seat while the back does not - an asymmetry. An object without asymmetry, such as a blank piece of paper, cannot have a (well-defined) front and a back.
EDIT Some objects have certain types of asymmetries which we choose to name front and back. Such objects must have sides. Sides are another type of asymmetry. An object such as a sphere has no asymmetries and so has no sides and no front or back. An object such as a cube has asymmetry which we name sides but no well-defined front or back (in the absence of other distinguishing asymmetries). An object such as a chair has asymmetry in the form of sides and other asymmetries in the form of front and back.
Perhaps I am misunderstanding the question - I say this because I feel my immediate thoughts on this question are different from the posted answers.
It seems to me that objects having a "front" or "back" is an arbitrary distinction that we impose on objects by convention for our own convenience and communication with others. I see two circumstances:
1) Front and back designate the part of the object in our visual field that is directly facing us (majority of the field) or away from us (none or only some of the field) respectively. One word was created to define the first case and a different word was created to define the second case.
2) Front designates the part of the object that relates to its utility, function, or standard; back designates the part that either does not or does so to a lesser extent. For example, the front door to a house is so defined because it faces the most common area of foot traffic in and out of the house and the way it is approached. The back door does not but it is still a door. So front and back can reference the usability of the object. The function of a chair is to provide a place to sit. We have designated this part as the front. The opposite side of the chair, where there is a flat, vertical surface does not allow one to sit on it unless one alters the original conventional position a chair is supposed to stay in (upright on its four legs).
The front side of a piece of paper with writing on one side is designated because there is information on one side, possibly of value. If there is writing on both sides, the front could be the side with a heading, a lower page number etc. That is the front because it conveys information about context - we do not read books backwards.
EDIT: Another example. In a notebook, the we put the whole punches/binding on the left by convention. So front and back is designated by the standard way the papers are organized. Again an example of function or standards.
1) and 2) are related because it would make sense to name the part of the object that is useful as what is generally in the field of vision. If you were demonstrating to someone the usage of a chair, you would not show the back of it to them - obscuring the seat - and say: "This is a chair and as you can see, you can sit on it".
EDIT: Another example. The front of human body is defined because the face is there. We have evolved to recognize faces and we identify people by looking at their face. So by function and utility, this becomes the front. And so on for other examples.
So it's a matter of function, utility, and convenience. I don't think objects have real front and backs.
Does this answer the question or have I missed the point?
The trivial answer is because of Euclidean geometry. If you designate one part the front (maybe just the part closest to you), then you can draw a ray into the object and whatever's around where the ray comes out is the "back". For example, this is the case when speaking of the "front" and "back" of a ball.
A slightly less trivial answer is because of geometry plus physics. Physical objects generally cannot simultaneously occupy the same space, much to the consternation of babies ("Waaaah I want to chew on my foot, my pacifier, and my hand all at the same time! WAAAAH!"). Objects also tend not to be packed one against another at high density. This means that only a small portion of a spatially extended 3D object is likely to come into contact with other objects. So it might be evolutionarily advantageous to have specialized parts for interacting, but there's no point putting them all over everywhere because you will probably not need so many. So the efficient thing to do is put them all together, roughly, where you can use them. That's the "front". The other side is the "back".
(I wanted to give an answer that might satisfy the niece, so this is a little simplistic.
The answer for adults is -- "anthropomorphism is ubiquitous".)
It is because we have a front and a back, and we like to think of objects as just like people whenever we can. Of course they aren't people. So the word does not really fit. We aren't even clear what we mean by 'front' and 'back' because we use them to mean opposite things.
When we talk about the front of a car, for instance, we mean the part that faces the way we most often face. But when we talk about the front of a book we mean the part that faces toward us. That is because we are thinking of the car as someone we 'are' and the book as someone we 'are listening to'. A desk or a sofa has a front like a car, a television has a front like a book. And some things, like an end-table, different people imagine different ways, and they don't agree on which is the front, but they usually think there is one.
Notice the word 'face'. It is all about faces. We put 'faces' onto the rest of the world because we want to interact with the world like we do with people, and for people, we focus on their face.
Cars, horses, theatres, televisions, shelves (well, most shelves), and most loudspeakers have fronts and backs (some loudspeakers, such as Magnepans, are dipolar). Tennis balls and tennis racquets do not. The distinction is the orientation of the functional structure. A tennis racquet can be used on either of its two sides. A piece of paper has two sides, neither of which is the 'front' until it's used. The side with writing on it then becomes the 'front' by definition. Some shelves have backs but some do not. It depends on whether you need access from both sides. If they have a back then they have a front, but if they have no back they have no front.
Imagine how having a multitude of mouths on all sides of your head would complicate life in general and brushing teeth in particular. This is why at some point creatures decided it was better to have a single mouth and then it made sense to generally always walk in its direction so they can say hello to whomever they meet. Ever since then everybody has a front side and a back side.
Because we have a front side and a back side, we like to make things with a front side and a back side, too, except maybe basketballs which we haven't figured out yet.
As others have pointed out, the concepts of "front" and "back" are imposed on objects by us, and are not inherent in the geometry of the object. Rather, the concept is a utilitarian one that looks at one aspect of how we use objects. In particular, if you look at the way we deploy this concept to objects, we designate the "front" of an object to be the side of the object that we face towards while we are "using" that object. Here are some examples:
Front of a car/bus/vehicle: The end that we face towards while driving.
Front of a TV/computer: The side that we face towards while watching.
Front of a person: The side that we face when we are having a conversation.
Front of a book/document: The side that we face when we start the reading process.
Inducing a meaning from these examples, we see that the "front" of an object refers to the side/part of the object that we would normally face/view first when the object is used. This concept is useful to us indescribing the shapes of objects by setting an orientation relative to our usage.
There is another angle on this, projective perception. We tend to see fronts, and our brain generates ideas about backs and unseen surfaces, which is held to some extent tentaively, in the mental map of our visual field.
We tend to think we are just seeing things as they are 'out there', but our brains do lots of things to help, like identifying edges, guessing volumes and whether things are concave and convex towards us, subroutines for identifying faces which underlie how we anthropomorphise things, and lots of things about guessing speed and location that help us cross roads through traffic safely or catch cricket balls moving too quickly for our eyes to see accurately. Optical illusions help us 'catch' our brain at some of the things it does outside of our awareness, like assumptions that lighting is from above and judgements from surface opacity.
We can look at how our projctive capacities develop over time, like Young Children's Knowledge about Visual Perception: Projective Size and Shape, and more generally. We can also look at the effects of brain injuries and surgeries, like split-brain syndrome which helps us understand how the right brain hemisphere is especially involved in creating the integration of a mental map of perceptions, while the left hemisphere is more associated with integrating perceptions of the self within that space.
In short, it is highly useful to imagine things we can't see, like faces of objects we can't see.
Your niece asks an interesting and perceptive question. Great physicists invariably have ways of relentlessly questioning even simple things we take for granted, like Einstein's thought experiment of riding on a light beam. Feynman specifically discussed why mirrors flip left-right but not up-down Let's hope your niece is showing early signs of being a great physicist!
Not all objects have a front and a back. You could hold a soccer ball up to your niece, and ask her to reliably identify which side of the ball is the front.
Sometimes we can even say something has a front but no back. We can talk of "wave fronts" in engineering, describing the leading edge of a wave, but we don't have a corresponding "wave back."
However, there is something to be said for the question. Alan Watts loved to say that the one secret to life is that where there is an inside, there is an outside. He said it many times during his career, so it shows that you can make a living as a philosopher and still ponder why things have these insides and outsides, or fronts and backs.
Here are some of the definitions from the Merriam-Webster dictionary for “front”: “a line of battle”, “an area of activity or interest”, and “the forward part or surface”.
These suggest that the front is the part of an object that faces the direction in which it is going or has an interest in going. If that object can have goals and intentions, such as a cat or a human being, the front is the part of the object that points in the direction of its intention or goals. This is similar to David Blomstrom’s observation:
If any object has a locomotion function, then front refers to the side that points in the direction the object travels.
Symmetrical objects, like a blank piece of paper, as Nick R mentioned do not have fronts. The object needs some asymmetry so that a special portion of it can be identified from other portions as the “front”.
But asymmetry is not enough. A rough stone has asymmetry, but no obvious front. The object needs some way to focus on a particular direction in which the front points. Nir illustrates the benefits of focusing:
Imagine how having a multitude of mouths on all sides of your head would complicate life in general and brushing teeth in particular.
This raises another concern. Not all objects that have fronts, such as a chair or a computer monitor, are able to focus themselves on any particular direction. These objects get their fronts indirectly when other objects have intentional, goal-oriented, future facing stances and use them. The objects with such indirect fronts can be thought of as tools. Those using the tools to implement their goals can be thought of as agents. As Ben notes:
In particular, if you look at the way we deploy this concept to objects, we designate the "front" of an object to be the side of the object that we face towards while we are "using" that object.
To better clarify what front is one can compare the pair, front-back, with other pairs, such as, left-right or up-down. How do these pairs relate to front-back?
Both left-right and up-down are relative spatial pairs associated with a frame of reference. But not any frame of reference will work. Does it make sense to talk about the left side of a ball? Even if one arbitrarily assigned a left side to a ball, one can rotate the ball 180 degrees and find the left side is also the right side or at least these two sides can be easily confused. Something similar happens to up and down.
However, if the ball or frame of reference is indirectly given a “front” perhaps by having a human being point to it, then one can identify the left side, the right side, where is up and where is down with respect to the front of the human being pointing to it. This suggests that left-right and up-down are pairs dependent upon front-back.
With that preliminary, consider the original question:
Why do things have a front and a back?
Things have a front to allow them to focus on where they want to go or what they want to do. If they themselves can't choose what to do they can get an indirect front by being used by someone who can.
For us, this is a trivial question. We identify agents and their front sides easily. From there we can identify what is on the right or on the left of that agent with a front. Automatic image annotation would attempt to find a way for a computer to do something similar that agrees with what humans understand as front, back, left, right, up and down. Making clear what one means by these concepts should help to verify whether the image annotation software has got it right.
Your three year old niece is one confused lady.
I assume that she is talking about visual "things", simply because I have never heard of the suggestion that sounds, smells, tastes or feels have fronts or backs.
In any case visual "things" do not have fronts or backs, because visual things are seen in one aspect only, ever.
So, I conclude that your three year old niece is not talking about things as such, but about the ideas of things. But even then, the notion that ideas have a front or a back is quite absurd.
Perhaps your three year old niece is a nominalist, and has never quite comprehended what Wittgenstein intended?