Most adults who wear adult diapers wear the diapers because they need them because the adult is unable to hold their bladder or bowels.
So the adult needs to wear the adult diaper to pee in or poop in and clean up later otherwise they would have poop pants or wet pants from peeing in them.
Some adults are elderly and cannot use the toilet at all anymore at least without assistance so it's easier to have them use a diaper and then change the diaper later.
Some older kids also need diapers such as my nephew who is disabled and cannot be potty trained so he has been in diapers since he was born.
He's now 7 years old and wears size 7 baby diapers because the adult and youth diapers are too big for him.
He wears Parents Choice size 7 diapers which work great for him.
Some adults also like to wear diapers which is okay as well because if the diapers give them enjoyment then that's good for them.