Are you referring to the plastic piece that must be removed to change the oil?
I believe it also helps air flow more smoothly under the car, less drag. It might keep some air from getting into the engine compartment, but not sure it would be a significant amount at speed. hmm.
I know its better to keep it on , it helps block it away from the weather , I had my oil changed and the guy forgot to put it back on , did not find out till checked my oil level could see the ground.
I did notice that the engine bay was not as clean and some rust on the belt pullys...