Answer:
Seat belts aid in the prevention of passenger ejection during collisions. They also restrain passengers into safer sections of the automobile in the event of a crash, as sections of the vehicle are designed to crumple to dissapate energy and the limbs of an unrestrained individual could inadvertantly travel into such sections and be crushed or severed.
Seat belts rarely ride down on a person when properly adjusted for their size. Belts should always be snug across the body of a passenger.