They reduce injury by preventing people by hurting themselves on hard objects, and prevent the impact from being so hard and shocking. Prevents less damage to the neck, back and head, if we didnt have these there would probably be more injuries because there wouldn't be anything to stop the passenger from falling forward into window, wheel, hard objects etc.
Airbags are and airbag
They can help by being a pillow when the car hits something or slams on brakes
Airbags absorb the energy of the passenger and immediately deflate to absorb the shock impact.