Some people just do not use their seat belts. There's a number of reasons people don't use their seat belts. Doesn't fit right. Auto makers are required to put them in, and you are required by law in the front seats to wear them. They are definitely life-savers.
Seat belts come in two major forms.
Shoulder and lap. The shoulder belt is used primarily in the front seat, and is meant to keep people against their seat in an accident, instead of flopping forward, possibly into the windshield. The other kind of seat belt is the lap belt. This seat belt is primarily used in the back seat, and is meant to keep people where they belong during an accident - in their seats, and not in the windshield. But before going on, let me make it clear that seat belts are a prime example of why this is an imperfect world.