People steal them. For what I don't know they can't me worth much
A few years ago i was at the rec at ku, and, looking out the big windows, i watched a car pull up behind a blazer, a guy got out of the car walked to the blazer and took the tire cover off the back. He got back in his car and drove off. His purpose is a mystery to this day.
I don't see the benefit of a tire cover, but I do see a hassle.
When I had stock tires my Jeep came with a spare tire cover. I had to change flats twice and it only added an extra step at the beginning and end. except for the second time, as I didn't put it back on.
I've since increased my spare tire size by 5+ inches, so the stock cover won't fit, and I don't see the value in spending additional money for no value.
Aside from the pragmatic/logical side, I prefer the look of the spare tire uncovered. Especially the bigger meatier tire that I currently have on the Jeep.
The only reasons I could see to use a tire cover is to block the sun & elements. Since the tires on the axles also sit in the sun, though maybe not as much but get more elemental exposier I don't see how a cover would really help. The only way a tire cover would help is if the same spare was used a really long time through the life cycles of multiple sets of axle tires. I don't need to worry about that because I change out the old spare with one of the worn axle tires when they are replaced. The tire carrier could actually keep in grime from offroading and do more damage than protection.
A tire cover helps with sun cooking the rubber of your spare, if you live in an environment where it rains at least 3 or maybe 5 times a year then you may not understand, in the northern deserts we get snow, but rain is rare, the tires on the ground are always being worn, as a tire should, the spare isnt, the top and upper side of the spare after just a few months of burning heat and sun get a glazed look and the rubber begins to get brittle, it isnt dry rot, it doesnt crack, it glazes. The only way to prevent it is to periodically turn the tire on the carrier, if your running anything much larger than a 33" or 35" that can be a chore, the other option is to cover it, the problem is its very difficult to find covers above 31".