Seatbelts do improve the safety for other people. Car crashes are not always just bang and then the rescuers come and maybe save survivors. A good blow to the head from being flung about can daze or knock you out, and you may lose control of the vehicle after that. If you're wearing a seatbelt there is a much greater chance you stay conscious and still have partial or full control of the vehicle after a minor collision. So hopefully it doesn't turn into a much worse one.

Here in Ontario, you can let your 14 year old drive around without a seatbelt, or a license, in a car on private property. But on public roads, seatbelts. That seems reasonable to me.