Footswitch toppers are colorful plastic bumpers intended to be put on the footswitch of your pedals. Some guys are using them for the "hard to reach" footswitches which might otherwise require a tactically aimed big toe to successfully engage without disturbing anything else.
Some people just like them as a bit of fun or as a way for them to pimp up their pedals. Practical or purely cosmetic purposes, it's entirely up to the user what to use it for....