It’s to do with Japanese conceptions of purity. There are areas which are thought of as clean and areas which are thought of as unclean. Thus you don’t wear outdoor shoes indoors. You have slippers, or walk barefoot indoors. Obviously the toilet is thought of as an unclean area, so there are special slippers you don’t wear anywhere else. Obviously you aren’t going to bring your outdoor shoes from the porch to the toilet, so there are slippers there for you to wear. You find them in private houses, restaurants, traditional hotels and rest-houses. It’s also a way of telling if the toilet is occupied or not. If the slippers aren’t there…. Also don’t forget to take them off before leaving the toilet!!! You’d be polluting the place and exposing yourself as a dumb foreigner who knows no better.