When you are done wiping your ass after taking a shit and you're not sure if you wiped everything away, the decision for a courtesy wipe has to be made. This is done as a courtesy to other people that might have to deal with your stank ass from not wiping enough.
Adam: "What is that smell?"
Mike: "I don't know. I don't smell anything."
Adam: "Did you just take a shit?"
Mike: "Yes I did."
Adam: "Did you consider a courtesy wipe?"
Mike: "What is that?"
Adam: "At least we know what that smell is now"