Damage can come from the weather, rocks, people riding bikes and debris from trees. Car covers can prevent or limit the amount of damage to your car. There is also long-term damage from the sun or water that a cover can minimize.
You can deter people from looking in your car or stealing when your car is covered too! It is easier for people to target a car when they don't have to fumble with a cover. Animals might be less likely to stow away if it is covered. Bird droppings will still happen but you can wash your cover instead of your car!