Silly question probably, but what's the point of an indoor car cover?
If you've got a collection of cars in a large hangar affair, you almost certainly get birds, squirrels etc getting in. The cover prevents their fouling damaging the paintwork.
If you have more of a workshop than a garage, the cover stops dust from all your other projects settling on the car, and again protects the paint.
If you have a showroom with high levels of sunshine, then the cover prevents UV damage.
And if like me you have a garage full of tools, bikes, the barbeque, more bikes, picnic chairs, old oil cans, rickety shelving and so on, and you carry jagged bits of metal perilously close to the car, the cover protects from scratches and other physical damage.
Having said all that, generally received wisdom is that the best place to keep a car not used every day is a car port, but again here a cover is advisable for most of the same reasons.