The entire purpose of a seat belt is to restrain you in the event of a motor vehicle accident. If you collide with another vehicle, your car is going to come to a sudden stop, while you will still be moving forward at a very high speed, unless your seat belt restrains you. Therefore, the purpose of the seatbelt is to prevent you from colliding with the inside of your own vehicle, which can lead to significant injuries.
For example, if you are driving a car, and you are involved in a motor vehicle accident, your body will still be moving forward unless you are restrained by the seat belt. This means that you could collide with the steering wheel at very high speeds, leading to rib fractures. The combination of your seat belt and airbag will prevent you from striking the inside of the car.
Typically, your seat belt is a combination of a lap belt and a shoulder belt. The lap belt restrains your lower body while the shoulder belt restrains your chest and torso. If you are sitting in the middle seat in the rear of the car, you might only have a lap belt, which unfortunately, is less safe, and can lead to major injuries.