These are surprisingly complicated questions, and I don't have the time/desire to touch on every consideration. But on the most basic level -- why do I blog?
A few reasons. My blog is a place for readers to go when they're looking for info about my books (which I'm not always the best at updating, but I try). It's a place for me to share cool stuff I've found on the interwebs -- because that's fun. It's a place to tell amusing stories, talk about writing, or rant about politics (because sometimes I can't help it). Sometimes it's a place for me to express deeper things -- feelings about life, and especially writing -- in the hopes that my words might resonate with other people, even be a comfort.
My blog also serves another purpose for me as an author: it's a line I draw. My blog is a carefully-maintained wall I put up, to remind myself of the difference between my private self and my public profile. It helps me define my privacy and protect my private soul, by giving me the opportunity to decide, over and over, what I will or won't talk about publicly. In a funny way that I'm having trouble articulating, presenting myself to the world through my blog helps me remember that it's really only my loved ones who know me. And I find that extremely comforting.