The "serious" answers get all sociological and some junk. We want to be heard popping off about our opinions on talent trees, bragging about one-shots, or feeling part of the larger WoW community outside our own servers.
However, as a WoW blogger myself, I know that the truth about us isn't nearly that pretty. We blog to brag about Kara exploits or show off our expertise on the Hunter class, a frame addon, or prime Oily Blackmouth fishing spots . We dream of demi-celebrity status in the WoW community. We try to provoke people into replying, sometimes with generous link love. We like to have the first scoop on expansion news (essentially, more showing off). We gossip about guild drama, sometimes wrapped up snuggly in the ignorance of our fellow guildies that we even have a blog. (There is a sweet freedom in finding the comfort of internet strangers.) We blog for artistic expression and to make others laugh.
And the main reason we blog? The same reason we feel itchy when we haven't called Mom in a few weeks: guilt.