The only thing I can figure is it's there to draw in cooler air from outside of the underseat area, kind of like the snorkels that lead from the air filter to somewhere out from under the hood.
For instance, my van has a snorkel that leads from the air filter to the front grill area. My truck had a snorkel that went from the air filter to the inside of the front fender, where the air is cooler. I can't see it making that much difference on a bike though