because it's a belt for a seat.
It is for your safety. More people without a seatbelt die in car accidents than with seatbelt. So I require to use the seatbelt. See here for more information about seatbelt usage: How_much_does_it_cost_to_replace_a_window
why it is ok to not wear a seatbelt
Buckle your seatbelt!
you are wearing the seatbelt.
The seatbelt
The word seatbelt has two syllables. The syllables in the word are seat-belt.
By putting your seatbelt on! Not wearing a seatbelt is one of the dumbest things you can do. Seat belts save lives.
a seatbelt = une ceinture de sécurité
By buckling your seatbelt.
Wear your seatbelt!
Can your license get suspended for seatbelt tickets how long
Put on your seatbelt.