To get to the other side, dummy. Any kid will tell you that, but actually there are a million different answers. I've collected some of them here. If you know of one which isn't featured here, mail it to me.
Plato:
For the greater good.
Nietzsche:
Because if you gaze too long across the Road, the Road gazes also across you.
Jean-Paul Sartre:
In order to act in good faith and be true to itself, the chicken found it necessary to cross the road.
Bhuddha:
If you ask this question, you deny your own chicken-nature.
Machiavelli:
So that its subjects will view it with admiration, as a chicken which has the daring and courage to boldly cross the road, but also with fear, for whom among them has the strength to contend with such a paragon of avian virtue? In such a manner is the princely chicken's dominion maintained.
Socrates:
You already know the answer.
Descartes:
The chicken thought "I cross the road, therefore I am."
The chicken's trajectory was described by the equation for a straight line.