For 1990-2009 cars

In moderate to severe frontal or near frontal collisions, even belted occupants can contact the steering wheel or the instrument panel. Airbags supplement the protection provided by safety belts. Airbags distribute the force of the impact more evenly over the occupant's upper body, stopping the occupant more gradually. But, airbags would not help you in many types of collisions, including rollovers, rear and side impacts, primarily because an occupant's motion is not toward the airbag.

Airbags should never be regarded as anything more than a supplement to safety belts, and then only in moderate to severe frontal or near-frontal collisions.