Here are Seth Godin and Tom Peters, talking about why they blog:
I always tell people not to worry about whether or not anyone will read their blog or comment on it. Just write. Write so that you can figure out what you want to say and how you want to say it. If you're good, people will read you. If you struggle, keep at it, and you'll get better.
Hat-tip to Ruchi Bhatia for the link!