I'm not sure if I'm answering his question, per se. However, when I look over my blog posts over the last 3 years I see I can categorize my reasons for blogging as: learning, reflection, reaching out, and recording.
I love to read blogs. I often think about what I read and come up with great ideas (in my own mind). However, my own family really is not interested in the same things I am. Blogging allows me to continue both formal and informal learning by working out and reformulating ideas that I have heard at school, questions from my students, read in blogs or the periodicals I read on regular basis (such as the Financial Times), and even analyze and come up with my own theories on what is going on in those areas in which I am interested.
Often, while I am thinking about what I will write, I think back to other posts or comments I might have made and go back to reread what I have written in the past. I rarely reread papers or articles that I have had published. But I do go back and reread blog posts, which often look different with time. This allows me to revisit ideas and build on them as I gain new experiences.
There are times when I just can't quiet my brain (usually at about 3:00 in the morning at the beginning of the semesters!). Blogging allows me to reflect on these ideas and put them into words. This requires a great deal of reflection and analysis, putting together ideas and verbalizing them. Sometimes they are just seeds of an idea that I am able to flesh out as I try to figure out how to word them.
This post is a great example. While I am working on a dissertation having to do with technology, writing and groups, I never thought about how to articulate the thought processes that setting up collaboratively written documents require. Without the chance to reflect, I would not have realized the level of learning that happens when I blog (see the section above). Blogging organizes my thoughts and quiets my mind so I can sleep at night!
This is perhaps the best thing about blogging. As I mentioned in my comment to Tony Karrer on a similar topic:
While I agree that the writing helps you to think, the difference between blogging and journaling is the public aspect of it. While no one may read your blog (I often feel this is the case with my own blog), a blogger is writing for that mysterious reader out there...for someone that might understand what they are saying. When you get a great comment on your blog (not something like, "Great post" but a question that you hadn't thought about or a different perspective) you have more than an internal conjugation of your ideas.
I love to read blogs and find blogs that address the issues I'm interested in. Whenever someone new posts on my blog, I click on their name to see if they have a blog. I figure if they are reading what I am writing, they probably are writing about what I am interested in reading about. I have found some great blogs this way. This networking makes the blogging seem more connected and less "personal." I feel that even if only one other person reads the blog, it is better than just writing for myself in which no one can challenge what I write, give me a different perspective, or give me additional information to support my ideas. I like the way the blogging connects my writing to others, something that I think is very important in writing (and writing instruction).
Finally, yes, as old fashioned as it sounds, I like blogging because it records my progress through life and learning. When I go back to my old posts, sometimes I'm really impressed with my insight; other times I can't believe my ignorance! I am a great believer in history, however. I feel that this is a record (as is everyone's blog) of my story at a certain period in history. I love rereading my father's letters to me during my teenage years when I was either away at camp or at school (college). My own kids are able to read them and get a sense of our relationship, his sense of humor, what was important for us as a family in terms of the world around us, in his own words. I feel a blog can be used the same way, making our thinking transparent.
Well, as I said, I'm not sure this answer's Ken's question, but his question got me thinking about why I blog.