If you don't wipe after peeing then urine can be left over especially if you're a women or girl and the pee could cause a UTI or rash.
As for boys there's usually no issue with not wiping after peeing since the pee comes out of a penis.
However if it's a baby or toddler then you should wipe them after a pee diaper as the urine can be in contact with the skin and cause a rash.