Viola!

the heading is actually to be read as, why law?
and the question is subsequently answered.
Black and white. Wrong and right. Yin and Yang. Everything , will in the end be flattened to the same level like it happened after the deluge in Mumbai. The wrong will definitely be punished, the law of karma is ever binding.
This brings me to a question Nick asked me about why i chose law. I don't think i was equipped enough to answer the question then - because i started to blame the law.'A bad workman blames his tools.'
But now i feel i can answer the question satisfactorily.

Justice is truth.
Truth is reason.
Reason is whose mercy upon which we thrive.
Not that a written code of conduct is to be strictly folowed, but law helps in guiding your life straighter lines, which makes for your participation in society. We are in every way connected to other people, no man is an island. Society and its opinions are our yardsticks, whichever side of the line we're on. (For eg., we might be 'bad' only because everyone else has a higher degree of goodness)

Now, why law?
I'll try to answer that indirectly. A situation was placed before me, wherein a poor man direly in need of money murders this man who's well off and takes his money. Nick said justice would be if the poor man was availed money by different means, not if he was put behind bars.
I couldn't answer well then, but here is a good idea.
We are all born as we are, in our families, to our parents, as a result of 'chance'. The nation (a nation is only an abstract idea, scrape away people, and their values and it doesn't exist) is not liable for a person's birth, fate and his standing in society.
The government is only the elected representative promoting order and sorting out administrative difficulties, and being a leader when the people need one.
In this particular case, a man lost his life due to another's misery. In such a case, the law requires that both people be treated equally, for in the end that's all it came to, one, armed; the other- defenceless, and their monetary status was irrelevant at the point of murder. The poor man's logic is irrefutable - but so is the rich man's (bless his soul) - I procured my money, through hardwork or by inheritance. It's not my fault that the man is poor.
A combination of logistics then, based on whichever is more oriented towards the general, wide-angle view of normalcy in society triumphs.
For in the end, we survive on the banks of sanity - so we can prevent random and protect people from harm.
Law makes its entry here.
Where we need an impartial party, because if there is agreement, there is bound to be disagreement by virtue of them being two sides of the same coin. disputes need to be sorted, upheavals nullified - and somebody that looks upon a situation and only gives it the value and attention it deserves - objectively.
In another instance, if one person slaps another with weak teeth, and all his teeth fall out, the slapper is only held liable for the hurt caused due to the strike and not the falling off of his teeth. Because weak teeth aren't normal - and normalcy accounts for justice.
And it isn't that normalcy doesn't comply with reason.
In fact, normalcy is an offshoot of collective reason, and derives its definition from it.
And that's why, i guess, law.
Deciphering the cryptic code, untangling the twisted web of the human mind - applying a logic as clear cut as math in everyday life and situations. The joy of reasoning...finally i did justice to the question!