In crashes, people need to be retained within the occupant compartment and not be ejected, as to reduce the likelihood of serious injury. Lap and shoulder belts play an important role in this. In effect, they retain you to the occupant compartment so you decelerate with it instead of slamming into hard interior surfaces. But not all belt designs are the same. Some are easier and more comfortable to use . This is important because a comfortable belt is more likely to be used every trip. Choose a vehicle with belts that comfortably fit you and your family, and wear them.
All new passenger cars have shoulder belts on inertia reels that allow upper body movement during normal driving but lock during hard braking. Some cars have a second lock, too. You can test this by tugging sharply on a belt. If it locks, it's dual locking.
Some vehicles have tensioners that activate in more serious frontal crashes to reduce belt slack which, in turn, reduces the risk of head and chest impacts with hard interior surfaces.