If you don't wipe your bum or butt after a poo then you can leave poop behind on your bum or butt.
The poop that is left on your bum or butt after pooping can cause you to smell and may lead to a rash because poop contains bacteria.
The best way to get poop off your butt after pooping is to use some baby wipes or a washcloth to fully clean the poop off.
Or use a bidet or shower wand to clean the poop off.
Also not wiping after pooping can leave poop stains in your underwear or panties as well and you can smell without even realizing it.
There have been times where I had pooped and then wiped but sometimes had a little bit of poop left over and it got onto my panties.
So now I use a bidet or at least a washcloth or baby wipes to clean the poop off my butt much easier.
Toilet paper does a pretty poor job of cleaning poop off fully but it's better than nothing.