A Frisbee is built in the reverse-bowl shape for a couple reasons. One of which is to act as a catch for lift. The rim creates a pocket of air slightly like a parachute, which catches any updraft or lift force.

The rim, as (extremely roughly) shown in the diagram above, aides in maximizing the lift generated be the Frisbee's motion. This provides a much more effective shape than a flat disc, or one shaped only as an airfoil on the top.

One byproduct of this design is an increase in the amount of drag generated during motion, which does slow the Frisbee during flight, as it isn't as aerodynamic.