Its not to make money. I don't get anywhere near the number of hits required for that.
There are a couple of reasons. Firstly content sesitive ads sometimes get it right. If one of link boxes presents a product someone reading the blog is interested in then its win-win as far as I'm concerned.
They are easy to ignore. There is a lot of research to back this up.
They are easy to block. Adblock Plus. Nufff said.
It is interesting to see what the adsense engine thinks people that would read my blog would be interesting. Currently on the meanderings front page the ads are mostly about anger management and racism. Neither of which I have ever posted about...