Blogs have one purpose. To show people that you have blog, to tot up the score in UG profiles, to bore, to be cool (wahatahatahatah! BLOGS ARENT COOL!) to say to your friend, "yeah well my blog says this, so there"...I'm sure there's other extensively researched areas that I missed, but seeing as I passed the "one purpose" a while back, I feel I must go to bed as it is very late here and I am starting to feel even more ashamed of my blog laden profile.
I like them. It's no different than writing in a journal, except other people can read it, and some people use that as a grab for attention, I do it because I have no life and am always bored haha. Sometimes I have important things to tell people and cba to tell each one separately, so I do it as a group thing. You know, it's not stereo, you gotta use it is hi-fi if it works.