Adults use diapers because of several different reasons such as bladder incontinence, bowel incontinence, convenience or they just plain love to wear the diapers.
Diapers are actually more comfortable than regular underwear.
I'm urinary incontinent and at first I hated having to wear diapers again.
But after about 6 months of wearing adult disposable diapers full time I came to actually love the comfortable feeling of the diaper.
Wearing diapers is very comfortable and even if I were not urinary incontinent anymore I would still wear the diapers instead of the underwear.
I can sometimes make it to the restroom to pee but I don't even bother because wearing a wet diaper feels pretty good to me.
It's okay to wear diapers for fun, for incontinent or just because.
Diapers are not just for babies, toddlers and kids and there's no age limit on wearing diapers.