It's nice to put a dress on your washing machine
Around here they put costumes on concrete geese!
Regarding trapping humidity, that's true for car covers, too, especially outside. An uniformed person might think something impermeable, like a vinyl cover, would protect a car from moisture, like rain falling from above. However, what such a cover does instead is trap moisture evaporating up from underneath, where it sits against the car surface and forms rust.