A couple other benefits in addition to what Larissa says:
1. You get a much better sense of whether you'd actually be a good fit for that agent after meeting/hearing them. (This may sound like a small thing. It's not. I can't tell you how many agents I would have considered on paper whom, after meeting, I wouldn't dream of working with for various reasons. To repeat an oft-quoted phrase here, no agent is better than a bad (for you) agent.
2. You're more likely to find out their preferred methods of handling queries, and can thus get faster/better attention when you do query.
3. They're more likely to ask for a longer partial at an event than in their online sub standards.
4. If you meet them, they're more likely to remember you and put a face/person to the query (or at least to remember the event, and, assuming it was a good one, feel kindly toward you for being there.)
5. You can hear more about their interests and tailor your query accordingly.
6. You don't have to squirm if you DO meet them wondering if they've already read your query and are trying to decide what to do with it.
None of this (except #1) is more important than whether they like the writing and concept of the actual work, but it can all add up to a certain advantage in the slush. IMHO, of course.
ETA: oops, cross posted with EECoburn. Same ideas, more words here, ha!