Anybody other than small children should use them! No duh. In a
lot of other things, I don't care if you do something stupid that can result
in your death or injury. However, if your car goes out of control for
whatever reason, using a seat belt can also help you stay in place, and
therefore maintain or regain control, and therefore avoid hitting my
car. Unsecured passengers flying/flailing about can also hinder the driver's
efforts to maintain/regain control. Small children (i.e., toddlers or
younger) are the exception, because they should be in child safety seats
instead, which are in turn secured in place.
(Some people tell anecdotes about incidents in which someone was saved by
not wearing a seat belt, or was killed or injured because they
were wearing one. These are extreme rarities, with their frequency
paling in comparison to that of their opposites. Listen to the news, and hear
how often they say something like "None of the deceased were wearing seat
belts. One passenger, who was wearing a seat belt, survived with minor
injuries.")