Boot covers protect your boots from damage due to constant use.  They come in different styles and materials from neoprene to nylon to fleece.  Some are made to be purchased for an exact fit (neoprene) and include a zipper.  These are usually chosen to not only protect the boot but for the highest degree off warmth.  Other covers are made of nylon, stretch up to 1- 1/2 times their size, still offer some warmth but are most often used to protect the boot from blade scarring or to have the boots match the skating outfit.