Slipping a tire cover over that rear spare isn't just something to do for looks. These soft covers actually have a measurable benefit designed to help that spare last for years and years.
Over time, an uncovered spare tire can accrue damage from the sun's UV rays which can dry up and crack rubber. Then, when you need the tire most, there is a good chance it will fail. Soft covers protect that rubber from sun damage as well potential damage from road debris. Plus, they are easy to put on and remove as needed. Most covers are designed to fit certain tire sizes and include an elastic edge for a secure fit around the tire.