Frank Paynter at asked about 3 dozen people why they blog. Their answers are at Sandhill Trek. I said:
I blog to be part of a community of people whom I respect; I want to understand their thinking and I want them to understand mine. I blog to be part of the conversation. I blog to remember. I blog to refine my thinking. I blog because I don't think I really understand something until I write about it.
But there are much better answers than mine in Frank's post. Go read it.