My oldest son started a blog a few weeks ago because he was bored at work.
My daughter started a blog as part of a college writing class. She actually got credit for writing about the Minnesota Twins.
My youngest son started a web site to post his digital video before there was YouTube and before blogging became ubiquitous.
Some writers I like have started blogging as a way to hone their writing chops and get some attention and maybe even sell books or plays.
So, y'know, why not?