Having used them in Super Pumas, Bell 212s and S-61s, I don't think it's a stupid question! In fact, when you change to a type without them you feel 'blind' when on the ground, where once you could look behind at the loading and refueling activity etc. They also allow you to look behind for signs of fire, if you want to confirm a warning.
The only disadvantage I know is they allow ice to acrete which may then fly off in chunks!