I've only been to 2 so my experience is a bit limited. But, what I do know is that provides the blogger with the fantastic ability to network with other bloggers. It's so great to actually meet someone who understands what it is that you do. Ask 1/2 my friends what I do and they'll stutter something about she's on the computer and writes stuff. Additionally, the conferences also provide great sessions where you can learn from the best. Great question and answer session with some pros too!
Ok, you really have me wanting to sign up for a conference...NOW!