I don't know about where you live but here it is illegal, which is hard to prove except if you are in an accident after which you are not able to do up your belt and the police find you were unbelted. I recall having this discussion years ago, and a woman was telling us of a time she was driving a van and she had lost control of the vehicle due to another vehicle cutting her off (or something happened) and she found herself thrown against the back door of the cargo van, when she was suppose to be in the driver's seat and the vehicle was still moving.<quoted text>
So then it should not be illegal for me to not wear a seat belt when I drive alone then right?
Had she been wearing her belt it might have kept her in the seat, and she could have recovered her loss of control of the vehicle.
Another reason it is enforced is that injuries in collisions are usually (with some exceptions) less when people are belted in, which the insurance companies want, and so with the power they have, to keep their costs down, they want you belted in.
When seat belts were first installed in vehicles, before they became mandatory, I once thought they were a nuisance and just got in the way, but after getting into the habit of wearing them for so long now, I don't feel like I can drive unless I do up my belt (sort of an unconscious feeling that I might fall off the seat or something). Kind of like not taking off in an airplane without the belt buckled.