as said above these a brackets to make the truck road legal. Because because trucks are now so tall, if you ever rear ended someone in a car, you could drive straight over the trunk, and crush the passenger compartment.
These brackets are fitted, so if that happens. These things will dig into the car, and stop the truck from killing everyone.
Same reason you see 18 wheeler trailers with those massive square tube brackets on the back. its so if you rear end them, you wont go straight under, and scalp yourself.
I actually find it funny because there is one vehicle that doesnt have to abide by these rules for some reason, and it is a vehicle that absolutely needs them, and that is the Yellow school bus!