From a natural rights' perspective, you may ask yourself: why the weak has the same right to life that the stronger has? And if that's true (that the weak has the same right to life that the stronger has), is there any benefit for human societies to respect and protect the right to life of any individual, either if one individual is weaker or stronger than other individuals?
But to answer any of those questions, you need to understand (or, at least, define) what is a right. And once you have defined what a right is, the next step is to find out which rights are common to all individuals of the human specie? (spoiler: in descending order of vitality, those are: the right to life, the right to privacy, the right to truth and the right to liberty).
I've found the answers (or, at least, my favorites answers) to those questions on the Cultural Liberty blog, by Crosbie Fitch.
To discover our rights we must examine our own nature, we must determine what power nature has given us individually, and how it is balanced among all individuals in equilibrium (harmony).
A natural right is an individual’s natural power in equilibrium. A right is not the power of a strong man to crush a weak girl, but the equal power of all individuals to protect their lives, their bodies from harm, their dwellings from intruders, etc. Thus, a strong man may have more physical power in his body than a weak girl, but the strong man has the same right to protect his body as a weak girl has.
(quoted from one of Crosbie's comments on his article Questioning Copyright)
But your question is still looking straight at me: what are the benefits of protecting the weak from the stronger?
In a gregarian society (as the human societies are), particularly on those where individuals are seen as equal (ie. egalitarian), and the rights of individuals are recognized, you may expect that society to look for and reach some kind of harmony, equilibrium, balance. You might intuitively feel that that equilibrium might get broken if we just accept that stronger people could do whatever they want (ie. to kill, to invade, to deceive, to slave) to weaker people, violating their rights to life, privacy, truth and liberty.
And here is the catch: the "this is not right/harmounious/balanced" feeling even applies if a little weak girl, while holding a gun, tries to kill or rob or slave the strong man. As you can see, "stronger" and "weaker" become very relative once we introduce guns (or knifes, or chains, or armies) into the equation.
So, once again, Crosbie Fitch shed some light when I asked him: "What is just and unjust? Where does the reasoning or feeling of fairness or justice come frome?"
The symbol for justice is the scales - because justice is about the maintenance of equality and its restoration or remedy in the event of inequality - the maintenance and restoration of balance.
Rights are a priori the equal powers of individuals, in balance, in equilibrium, in harmony. Thus, should one person not respect the boundaries of equilibrium between them and their fellows, and overstep them, then they have created an imbalance (inequilibrium), and eventually, if this is noticed, those who notice it will probably seek to remedy the imbalance and/or restore the balance - to maintain social harmony.
We intuitively recognise when boundaries of equilibrium are breached because it is easy to see when another person seeks to be superior to another, to assert more power for themselves (physically or opportunistically), and more of what follows as a consequence, than if they were equal. We know what would happen if people were equal, and therefore what is just (fair), and that if something has happened through inequality, it is unjust (unfair).
We may well recognise that another person is more physically powerful than ourselves, but we know that enabling people do to what they will based upon brute force (or opportunism or guile) is not a civilised/harmonious/egalitarian basis on which to socialise.
We therefore seek justice (restoration of balance) when one person violates the rights of another, e.g. injures them, burgles them, defrauds them, kidnaps them, etc.
Injustice is when justice cannot be obtained, or when those in a position to provide it fall short of doing so, or even make things worse.
Injustice is also when legislation (established judgements or instructions in the provision of justice) is made that make things worse, that effectively make some people superior to others, i.e. give powers to some at the expense of others.
As Thomas Paine observed:
"It is a perversion of terms to say that a charter gives rights. It operates by a contrary effect - that of taking rights away. Rights are inherently in all the inhabitants; but charters, by annulling those rights, in the majority, leave the right, by exclusion, in the hands of a few... They... consequently are instruments of injustice ... "