It's a good question that has been put to me at couple of time recently. I've got a few answers.
First, for me, the reason I got into blogging was as a technology experiment. I wanted to play with blog-related technology so that I could understand it better. It's clear that I am not a writer. I do not enjoy writing. I find it one of the most difficult activities one can undertake. So much of human language is so vague. I prefer computer languages. ( As an aside, I'm one of those people annoyed by the addition of ginourmous to the dictionary. The point of ginormous was that you were describing something so large that the existing words for large were insufficient. Now we have to use something else. Hugantic? Collosatitan? Googantic? )
Reason number two was as a social experiment. My social skills are limited. Some people are energized by interacting with people. I find it draining. One on one, ok. Dealing with 2-5, fine. But the more people the less fine it gets. I thought that by blogging about various interests I might meet other like minded folk. Aside from a few very limited examples, that failed. Lesson one - blogging is not really a social activity. It's related to social activity, but even with comments, it is really more of a unidirectional form of communication. I'm not the kind person who writes letters to family and freinds, but this does offer a glimpse of what I'm thinking to those that want to know without my having spammed everyone.
Reason three - catharsis. This wasn't a driver originally, but I've found that it feels good to throw ideas out into the ether. Maybe someone will find it. Maybe not. But it's out there. A minor contribution.
People have suggested that it's to get a job, or part of some ego trip. Not really. I don't think anyone would hire me based on the content of my blog. As for the ego component, I imagine there is some truth to that, in an arm-chair-quarterback/back-seat-driver sort of fashion.