There is the useful reason: I enjoy knowing what my friends are up to, what's on their minds, and twitter lowers the technological barrier to that. The converse is true, and so I quickly hammer out a snapshot of my state of being, putting no thought into it.
Then there is the vain reason: I am interested in eliciting some specific perception from my friends. I have even a broader scope: I can elicit a perception from the readers of my blog, and from my 400+ facebook contacts. In other words, I use it as a sort of platform, but when I try to stick to the "What are you doing?" question, it is a platform thats all about me.
Then there is the lazy reason: This manifests itself rather as a form of loneliness. My thoughts swirl about me, and I recognize my need for friendship. I want to reach out and open myself to other people; in fact I need to. Everything I say on twitter is something that really belongs in my everyday conversation, and the frequency and time in which they take place indicates a void in my social existence.
These are just a few directions that twitter can go in.
It is something to be able to paint a particular picture, or to carve a statue, and so to make a few objects beautiful; but it is far more glorious to carve and paint the very atmosphere and medium through which we look, which morally we can do. To affect the quality of the day, that is the highest of arts. Every man is tasked to make his life, even in its details, worthy of the contemplation of his most elevated and critical hour. - Henry David Thoreau