Because humans are fundamentally an imperfect, and often flawed species. We're not really good at being good to each other, particularly when we have power over another.
Don't get me wrong, there are a plethora of wonderful individuals out there, but there are monsters too.
So we, if we are to live in a society, a collection of individuals need to make a choice. If we are to be realistic and accept the fact there are evil people in the world, we must do what we can to prevent evil, and maintain as much order as possible if all good people are to thrive.
So, what we've come up with is "rule of law". Our laws are imperfect just as each of us are. Their application and use is as imperfect as we are as well, but it's the best system we've come up with for maintaining order, which gives the good people of the world the environment needed to have a fulfilling life.
Laws don't dictate morality, but morality should, at least on some levels, dictate our laws. Equal application being of utmost importance.
No. (Well in cases of self defense it gives the victim the right to protect themselves, but I'm trying to ignore gray areas for the sake of discussion.)
It gives society and victims the duty of punishment. It requires us as a group of individuals to try and protect future innocent good people from falling victim.
We must do something to the violator to prevent, as best we can, future violations.
Again, it's like scolding your kid.