I'm sure some people do wear diapers to concerts so that they don't have to leave the concert to go pee and instead use the diaper.
I've seen diapers peeking out of peoples waistband in the rear when the persons shirt has rode up but I never though much about it.
I've never worn diapers to concerts though because I can hold my pee in until the concert is over and then I go right to the toilet to pee.
So for me and my strong bladder no diapers would be required.
However diapers could be good for some people who have weak bladders and cannot hold it in until they get to the toilet at the concert, movie theater or other event.
Adult diapers are sold more than baby diapers so there are adults using adult diapers and there's nothing wrong with it.
I personally think it's a good idea for some people to wear adult diapers to concerts as it would be very convenient and it would be better than having wet pants.