I take a slightly different approach when driving my car. Unless the car is clean enought to cover it, I don't drive it. Here is my theory:
1. The cover deters theves because it takes effort to uncover the car and the steal it, or its contents. My assumption is that theves are lazy.
2. The cover prevents people from keying my car or vadalizing in general. Once again those who do these things are probably not the most meticulous people so why key my car when there is probably another nice car they can key and NOT have to remove the cover.
3. The cover prevents *minor* dings. I mean minor. It gives an extra layer of protection when people open their door and let it rest on your door while not actually making a heavy impact.
4. The save the interior thing the other person mentioned.
5. Two words: Bird Crap.
6. Another two words: Tree Sap.
None of this answers your question though, ....but really,.... it does. There is a TON of dust in most people's garage. If you leave your car in the garage for a week uncovered and then go and cover your car when you are out and about, you will impart scratches on you paint. Covering it in your garage just allows you to cover your car when you are out.
I have a bunch more reasons but I don't want you to think I am as anal as most of my Sacramento Club members already do.
Long post, simple answer,.... Can you tell I really don't want to be at work?