Unless you’re my grandmother (Hi, Grandma!!), you probably don’t use the shower cap provided at some hotels for anything whatsoever. But, that little plastic cap with the elastic ring can be very useful when packing your bags.
Shoes are often cumbersome and tend to get in the way when trying to maximize your luggage. Take the shower cap from your hotel room (or one you’ve previously stolen from a hotel room) and open it up. Put your shoes inside it and the elastic will hold them together, making them much easier to handle and pack/unpack.