First of all, its not a seal, its a wind deflector...there is one on almost every car that has a moonroof. It serves two purposes.
One is to let air in your car...if there was no lip, the air will just pass over...so why not just leave the damn thing closed. In order for air to enter the cabin, there has to be a disruption in the aerodynamics of the top of your car.
Second is, depending on weather, speed, and how many windows are open, the wind passing over the top of the opening can cause a vacuum that is unpleasant to the ears.
I only know this because I held it down one day and happened to be going just the right speed for it to make the pressure in the cabin fluxuate and bug out my ears like I was flying on a plane.