I would imagine that a car left standing would have dampness problems. I'm guessing that everytime you drive your car, the air movement around it helps to dry it out and a car left standing will not be able to get dried out quickly and wet/damp areas will form, particularly in places where the water will lie.
My dad used to say that cars left in garages rusted quicker, for a similar reason. If you put it away after driving in the wet, it will be in a garage which will become more humid and again the car won't dry. At least outside there is some wind and fresh air.
I think that is why keepers of vintage cars go for specialized equipment in their garages to control the humidity.