I blog because sometimes, I have things to say. Things that I don't want to say in person, just because I never remember to or it never fits any conversation quite right.
Of course most of what I blog came up from conversations, so really I blog because I want to share interesting things I've said and been told. And when what I want to share goes beyond the regular limit of a facebook status, or perhaps I just want to share it with the internet in general, that's when a blog is needed. And to share that with friends would require much less time since I wouldn't have to rewrite everything for everyone I wanted to tell the same story to. Copy and pasting links take far less time.
Sometimes just writing to relive the story or to expand on a thought is fun. It's like writing in a diary, but not a private one. It's this strange pseudo-privacy that only the internet can give.
And sometimes when I'm feeling bored or down or have nothing really better to do, it's nice to just sit down and write whatever comes to mind. It's more for me, but if it were only for me then it'd stay in my head. Things always become more real when it's no longer just in your head.