I write for several reasons.
To me, you can't really exist without writing. It's a way of adding your chapter to history, a way of living on long after your time is over. Writing gives your perspective on life and makes it possible for you to be remembered, even when your era is just a brief age in history.
And I write because it reaches the world. I can say more by writing than I can by standing in a public square and screaming my beliefs. If I write something, odds are people are going to pay more attention to it than if I walked up to them and told them myself. People take writing seriously.
My last reason is that you can express yourself when spoken words fail. When you're so downtrodden that you can't even speak, you can write. When you're too happy for verbal expression, you can write. It's an art form of portraying the emotions that your body can't physically describe.
In short, though, I guess I write...because I exist.