The internet is a really huge place, and a lot of people are doing a lot of things for many reason, I can only speak for myself:
For me it's for the same reason that I make art or talk to people - to create and interaction and to express myself as an individual person. I think that one of the things in common to almost anybody in the world is a will to be heard, to be listened to or seen. I think that a lot of people express their inner thoughts or feeling in different frames - may it be in a therapist office talking about feelings, or in a political debate at lunch break or by writing a diary or spray painting their neighborhood.
What I like about contributing to the internet is that it exposes my writing and art, and mostly, myself, to a variety of people who would have never seen me or it and also that the boundries between a viewer, an artist and a participant are not as clear as in the offline reality. I really like the way the the internet incourage action (n commenting or responding) in a large social group. while other communication tools are either very personal (phone call, conversation with a friend, diary) or very passive for the user (like TV, radio or newspaper.