The safety belts hold you in place if your car crashes. It prevents you from banging your head on the windscreen and/or bleeding to death.

When a car immediately comes to a halt, due to your body's inertia, you keep moving forward and might hit the windscreen or dashboard. But the seatbelt holds you on placeand minimizes the damage caused.
As we know that when the brakes are applied, we move forward.This happens because of our Inertia. If we do not wear seat belts then we will hit hard on the front of the car.This will result in an accident. So, safety belts are very very important for us.