I suppose filling a blog with all your personal thoughts about this and that, is as much a personal escape valve mechanism as it is a platform for your 'diary' or personal life story. We may well share the funny stories and gaffs of our working lives but for many, blogging is borne out of sheer frustration. The blog becomes a personal soapbox and a megaphone for those political and social issues that affect/annoy us.
My motivation is probably a combination of all the above and, if nothing else I find it relaxing, even when I'm blowing a fuse about the latest hair brained government initiative for the police! I suppose every blogger would like to think people have read their toughts however, I suspect the outpouring is more important than how it is /is not received?
WikipediA: A soapbox is a raised platform on which one stands to make an impromptu speech, often about a political subject. It is also used to express concerns or to release frustration. A modern form of the soapbox is the blog.