Seat belts are designed to keep you in your place, so that as little contact as possible is made between the occupant and vehicle interior.
They prevent and/or reduce injuries suffered in a crash. They keep you from being thrown out of the vehicle or from getting crashed by the board or the airbag. Remember, when there is an airbag,always wear the seatbelt. Because the airbag opens at a speed more than 250 km/s and might kill you in case of a crash.