Very simply, if you care about the way your car looks, you need a car cover. Even if you garage your car, a car cover keeps it from becoming a dust collector and protects your car's finish from accidental scratches and dings. Dust may seem innocuous, but it can actually act like a fine abrasive and leave scratches in your paint. When you consider the clear coat on the average car is only 0.0015-0.002" thick from the factory, you don't have a lot to work with to polish out those scratches. If you're like most of us, the garage is also a workshop and the car has to share space with things like lawnmowers and bicycles. With a car cover, if you're working on something and accidently splash lube or cleaner, your car is protected. Outside, of course, the need for protection is even greater. Nature and the elements are constantly assaulting your car's finish, and the chances for dings and scratches are even greater. Bird droppings, tree sap and tree fruit like berries can cause permanent chemical damage to your paint if left on the car long enough. Dust, pollen and airborne dirt can all abrasively damage your paint. UV rays can oxidize the clearcoat and fade the colorcoat. When combined with rainwater and air pollution, sunlight can also cause water spot etching.
Your interior is also subject to UV damage and fading from sunlight. Although auto glass filters out some types of UV rays, some still get through and can cause damage to exposed surfaces. Plus, the heat from the sun can cause the greenhouse effect inside your car, with intense heat drying out vinyl and dehydrating leather to the point where oxidation and cracking occur.
A car stored outside also gains theft protection with a car cover. Just having a cover over your car "keeps honest people honest" as the old adage goes. A casual joyrider isn't going to waste time trying to get a car cover off. And unless you have a cover that advertises what's underneath, a car cover can conceal the make and model, making it less of a target for thieves looking for a certain type of car. A car cover also keeps items inside the car away from prying eyes. "Out of sight, out of mind" helps keep opportunistic thieves from making an impromptu break-in to steal a stereo or other valuables. Most car covers also have grommets where cables and locks can be attached for even more security.
So, you know you need a cover, but what kind should you get? The first consideration is getting a cover made of the material that is most appropriate for where and how you store your car. Covers for indoor storage can be lighter and simpler because they don't have to deal with the weather. If you use your car on a regular basis you'll probably want a lightweight cover that is easier to remove and install. For long term storage a heavier cover can offer additional padding for impact protection.
There are a myriad of outdoor covers available, and these should be selected according to the type of weather that is most often encountered as well as how often the car is used. If you live in a rainy humid region you'll want a cover that sheds water, dries quickly and is breathable so moisture can escape. Areas with intense sunlight will require a cover with maximum UV protection. Just as with an indoor cover, if you regularly drive your car you'll want a cover that is light and easy to handle, whereas a cover for long term storage can be heavier.
Once you decide on the type of cover, the next decision will be whether you get a cover that is custom-fit for your car or a universal cover. A universal cover is a cover made for the "average car" that is approximately the same size as your car. It won't have pockets positioned where your mirrors are or for your antenna, and there won't be any allowances for any other unique features on your car. And as mentioned previously, a universal cover is made for an "approximate" size car, which means it may fit your car either too tightly or too loosely.
A cover that is too tight may put undue stress on parts that stick out like mirrors or spoilers. Over time, this stress may cause damage to these parts or to the body where they're attached. A cover that is too tight may also be difficult to remove and install. An outdoor cover that is too loose can be particularly damaging to your car. Since the cover doesn't fit tightly, the wind can get underneath and beat the loose cover against the body, damaging paint and trim. The wind can also blow sand and dirt underneath the cover onto the body, then the loose cover can slip and rub against the body, grinding the abrasive material into your paint. On particularly windy days, unless lashed down by additional means, a loose cover can completely blow off the car.
A custom-fit cover, like Coverking or Covercraft, is specially made for your particular make and model. The fabric is cut to match your car's dimensions and additions like pockets are provided in the exact position where your car's mirrors are located. Additional accommodations for things like hood scoops, spoilers, wings, grille guards, etc. can also be provided. In short, a custom-fit cover will fit your car like a glove. A universal cover will usually be less expensive than a custom-fit cover. But initial low cost is false economy when you consider how much damage a universal cover can do to your car's finish. A custom-fit car cover made of the appropriate materials will provide your car with the most protection.