For me, writing is almost a way to say things that otherwise wouldn't come out. I'm thinking that many writers were able to say loads more with a pen than what actually came out of their mouths. This holds very true for me as well, i've come of as almost a different person when i go back and read the papers I've had to write. Another idea of why we write, and i'm not sure if anyone's touched upon the notion yet, is that for many people, myself included, actually conveying what you mean is not an easy task. Writing things down provides a visual aspect to the spoken word, and it allows clearer thinking and perhaps clearer understanding.