Once upon a time, air bags were nonexistent. Now they are a standard feature in all new cars. This became a requirement for cars in 1998. When you get an accident at a high speed where the vehicle stops quickly, we know that the person inside wants to keep moving in the same direction at the same velocity as before the crash. Seat belts help to prevent the body from slamming into the wheel or flying out through the windshield. Your head can still fling forwards. The air bag deploys in order to slow the person over a longer period of time. It "catches" the head (or body) and brings it down to a slower speed over a greater period of time so the overall impulse (impulse= force*change in time) is much smaller. A higher impulse force on an important part of the body, like the head, causes injury or even death.