No one needs to, it’s just that extra layer between you and the outside world. And trust me, I’ve gotten skin caught in a zipper before and there are certain parts of my skin I’d NEVER want to get caught in a zipper (I think of that scene in There’s Something About Mary…“BEANS AND FRANKS!!!”). Yes, it’s cleaner, yes it keeps you in place, yes it prevents “leakage” of multiple varieties, and personally, after a lifetime of wearing them, I just feel “wrong” if I don’t wear them. But there’s no “law” I say do what feels right to you.