The greenhouse under the fabric
A car parked on tarmac or gravel sits above a continuous source of water vapour. Overnight the panels cool below the dew point and that vapour condenses — on the underside of the cover if it can escape, on the paint if it cannot.
Under a sealed cover the water has nowhere to go, so it cycles: condensing at night, warming during the day, sitting in the shuts, around the badges and along the trim. Months of that does more harm than rain ever would.
How a membrane solves it
A breathable membrane has pores far smaller than a liquid water droplet but larger than a water vapour molecule. Liquid rain cannot get in; vapour can get out. It is the same technology used in technical outerwear, scaled up.
How to keep it breathing
Contaminants block pores: road film, tree sap, detergent residue. Wash the cover with a dedicated cover wash a few times a year, never with fabric softener, and reproof the outer face when water stops beading.