To get out of the slush pile. It's as simple as that. There is no quicker way to advance to the pile of solicited manuscripts than to make a personal connection with an editor or agent. And that is what it's all about. No matter how wonderful your book is, it's no damned good at all unless someone reads it.
Conferences are fast-tracks for making that happen.
For example, if you write westerns you might want to go to the Western Writer's of America conference. Nearly every genre hosts a specialty conference and admission is open to anyone with the registration fee. Some of the best are regional conferences, where representatives of almost every genre can be found. University sponsored conferences are my least favorite. They often have impressive lists of attendees, but there is a definite in-crowd ambience at many of these that seems to defeat the spirit of a good conference.