Two factors influence the flight of a Frisbee, gravity and air. Primary purpose of the rim is to create an airfoil with a deep curvature. As air flows over the top of the Frisbee, it speeds up and the pressure drops, creating lift.
This is why my avg. pull in a game of Frisbee during the summer can be 80 meters or more, because the Frisbee is generating its own sustained force combined with the force I applied.