Seatbelts prevent you from becoming a missile. Not only can you hurt others in the vehicle, but the injuries to yourself will be much more severe if you impact the ground or vehicle as a moving object. Seatbelts themselves can cause injury, but it is generally accepted that the injuries are much less than the alternative.
The seatbelt is not designed to keep you from getting injured. It is designed to keep you inside of the vehicle during an accident. Earlier versions were called the lapbelt. Most deaths in auto accidents were from the person becoming evicted from the vehicle and then the vehicle landing on them.