You blog as a social activity, and in particular because it's an easier one than most to fit into busy days. It gives you a way of interacting with a world outside your own, a world which, as Heather initially pointed out, is unusual in its courtesy and its sense of welcome and inclusion. If you work from home (as I sometimes do too), blogging provides comradeship and a wider arena of engagement.
You blog as a creative outlet, because you like words and images. Because it's a constructive use of idle time.
You blog because it's fun and escapist and makes you smile. Because it gives you the adult equivalent of pen-pals. Because it's undemanding, in the sense that you can post and comment or not as you wish, with no sense of obligation.
You blog because there was a 'Create Blog' button.
You blog as a way to organize information and images, or as an adjunct to a business. To have something other than work into which you can channel a sense of enthusiasm and a work-ethic.
I had such a sense of recognition as I read all your responses. I blog out of megalomania, of course, but also for the sense it gives of community and of an external world. The craft/creative element of the blogs I read gives me a connection with the time before I was a gadabout or had children when I put into practice the knitting, sewing, and other skills my mother took the trouble to teach me, and encourages me to apply those skills and to be more creative myself.
Because blogging is slightly less real than reality, it gives me a sense of order. When Blue Daughter eats Pink Daughter's To Do list and mayhem ensues, I can escape to Google Reader, where no one must Stop That Right Now Or Else.
And it gives me a more personal sense of the world. We all see the daily international headlines about natural disasters, political crises and human drama, but blogging creates for me a parallel and no less valid world where Australia is reupholstering a chair, America is expecting a baby any day now, and Canada has a lovely pair of shoes with velvet ribbons.