I won't presume to know any ones reasons for blogging, as I'm not sure I completely understand my own. Instead I'll attempt to explain how I got started and maybe do some reflection along the way. ;)
I recently got started in computer science and was agitated that we had not done any stuff on the internet. I decided to take measures into my own hands and start up a web page. We have space given to us at my college, and it seemed a good learning process. So, off I went.
During the same time I had also moved away from home and gone through a rough break-up with a long time girl-friend. I didn't know anyone at my new college and was needing a form of expression. I had read someone's blog in the past and had been very enthralled with it. It was that blog that even put the idea in my head.
I found that, for some reason, it was very easy to say exactly what I felt when putting it on the web. I knew of course that very few people would actually read my work and those that did would be family and close friends. One would think that knowing the people you're closest to you would be the ones reading it, would severely affect the honesty of posts. I've not found that to be the case for me. I, of course, am somewhat careful of the information of others I give out, but normally feel comfortable rambling on about whatever boring and/or personal issue I'm dealing with at the time.
So, I guess the main reason I do it is a sense of catharsis. I have a tendency to ramble on about things that are sappy, convoluted, and/or boring. I don't like to talk about such things to people directly often. I find myself rolling my eyes at people (inwardly of course) when they talk about similar subjects. Perhaps you know what I'm talking about: the people that say, "Have you ever thought about *insert some inane or even common occurence* like *insert some off the wall or overused conundrum of an idea*?" Or, "Let me get you to think about something for a minute..." It's the attitude not the ideas that are annoying, but I find that such subjects have come to imply that attitude. So, to avoid all that, I say it on the web. That way, I'm not caging someone into a conversation they'll hate, but people can still listen if they so desire ;)