A cat is a cat. By why are this cat and that cat equally A Cat?
Indeed, there is a natural inclination to be impatient with philosophy, and certainly Plato's concept of the Forms does take some mental grappling. Why bother? First, so you will know what other people are talking about when they talk about Plato. Second, by wrestling with such ideas you can expand not only your knowledge but your imagination, the ways you see things and the points-of-view your brain can assume.
As was pointed out by others, the Form Cat is that which makes the specific cats the same sort of thing and not some other thing. All the instantiated cats are "part of" or, to use Plato's peculiar term, "participate in" Catness. How and why this concept becomes possible is not quite as simple as just "naming things." It may seem physically obvious with cats, but, as Plato explores in The Republic, what about "acts of justice"? Or things we call "good"? What is the Form of "The Good"? How can so many entirely different things assume or "participate in" this... Form?
And as noted previously, Plato draws on geometry as his paradigm. The triangle is not what you actually draw. Nor does it depend what you draw it on, or its scale, or how many times you draw it. It remains "triangular" or within the Form of the triangle. Plato is struggling to grasp the same kinds of truths we grasp in geometry, though in all the "roughly drawn" and sensed objects of the world.
Another metaphor employed by Plato is light. Today, we say that light enables us to see things. What we "see" is not neurons nor is it the "cat." We see the light bouncing off the cat and exciting our neurons, or so we put it. But in fact we never see Light. We only see the partial refractions, bending, and distortion of Light that we call "things." When you see the cat, you see a partial, temporary rupture in Light. The Forms also have something of this quality of "being partially revealed" by matter.