Never thought I would say this sentence, but "Lucifer has it right." By getting off your seat, you move the center of gravity low enough, without having to lean the bike as far. Try it for yourself. Oh, and if you've never ridden a dirtbike before, this may really screw with your head.
Find yourself a nice broad sweeping corner, that you can take at a comfortably slow speed, but still be fairly leaned (those looped entrance ramps to the freeway work well). Halfway through the corner start sliding off the seat, but maintain your turning radius. You'll notice that you naturally reduce your countersteer, and the bike stands up a little bit to balance you hanging off the side. Or do the opposite, come into the turn leaned, and slide back onto the seat. You'll notice that you have to feed in countersteer to maintain your radius. (For those of you who bodysteer, and haven't figured out countersteering yet, don't try this. We don't want to have to scrape you up.)
In the end, this keeps the bike more upright, and keeps more rubber on the road, also the force Normal to the contact patch is kept closer to perpendicular to the road. All of which means more traction.
Also, it makes it easier to slide your knee, which looks really cool.