I never even dreamed of using a cover, and before joining this forum I never really thought about it, but having seen many threads about it over the years, and considering the pros and cons, I still don't believe using a storage cover has any major advantage except if the caravan is stored under trees, or inside where birds may roost or if there is some other activity producing materials that might attack the paint or windows.
After all you don't cover the caravan when your using it to protect it from the weather, so you shouldn't need to if its in clear outside storage.
If you use a cover there are a couple of things you need to consider:
It needs to be secured to prevent it from flapping or rubbing on the outside surfaces of the caravan to prevent it damaging them.
It does need to allow the caravan to ventilate, if the caravans humidity cannot escape properly even in the winter months on some days the sun can still provide a lot of solar heat gain, that will raise the internal temperature turning the inside of the caravan into a greenhouse, and making ideal conditions for growing moulds etc.
As others have pointed out, you need to be able to fit and remove it, and be able to store it when you're not using it.
Quite a number of covers in have seen, have started to look quite tatty after a couple of years, not only faded colours, but stitching begging to fail, fabric suffering from UV degradation. I do wonder how long they will last before needing repair or replacement.