Why blog?

Having browsed quite a few blogs, I am not sure that I get it. Is blogging just an excuse for some people to self-publicise?
Some blogs clearly have a theme based around interests, and exist to share ideas about cricket, or boating, or film, or birds. I like the ones for which I share a passion. Some seem to be rather more self-obsessive, and are often rather dull. Yet another group clearly come into the category, "I have this idea that I may be able to cash in on as a book." These usually have a diary theme, or a year of doing something. You can cook your way through an American book on French cookery, for example, in one year. (Can anyone tell me why she didn't cook her way through Escoffier? No, I know the answer already, people wouldn't have read about real French cooking. Sigh!) Some of these blogs are fun, and I read, or listen to them from time to time, "The Song a Day Guy" is just one example.
So which is mine? Is it a bit of self indulgent rambling through my thoughts? Hmm. This blogging thing is more complex than it first appears.
In the meantime, here is a link to a blog I really enjoy. It is from Tim Erenata, a storyteller in California.
http://storylabx.tumblr.com/