Do seat belts really save lives?

It's one of those times where not wearing a seatbelt could save a life such as driving your car into the water and needing to get out. The main point of a seat belt is to make sure that if you crash or start flipping, that you don't get thrown out of the vehicle. Because the semi fell on the truck, there wasn't really a reason to get thrown out or flipped around. But in my opinion seat belts do save lives