Dude there are many different helmet designs, with both thick and thin plastic covers. In a practical day to day use case, the plastic shell is there for appearance as well as protection for the primary means to prevent damage to the foam core of the helmet. The foam is no good if it gets damaged the first time you accidentally drop it on the pavement.

I’m sure it can play some role in protecting your head, but it’s main purpose is to keep the foam on your head on one piece in case of an accident.