It stops the oil and bits flying out the back should the engine suffer a catestrophic failure. In front engined cars it is IMO essential as this will all go straight to the front screen. In a RWD beetle I'm honestly not sure if it is vital or not?
It might even keep the rain and damp out. Which seal is it? The top of the engine bay, around the opener or to the sides of the engine in the bay itself?