The reason to have your car detailed—or even washed, for that matter—is the same reason you comb your hair or wear a clean shirt: to look good.

You might also consider spending your money on detailing to preserve your vehicle’s finish. But having your car detailed—or washed—is not like changing engine oil. Fail to change the oil and your car’s days are numbered; fail to detail the car—or even to wash it—and you might lose a little resale value down the line (but probably not that much).

Removing salt, acid-rain residue, sap, bird droppings, tar, and other types of dirt from the finish undoubtedly reduces the damaging effects of these materials. Applying a protectant, as detailers do, adds a barrier against the effects of future deposits and makes them easier to wash away. Cleaning your car’s upholstery and carpets, like cleaning the carpets in your home, gets rid of dust, reduces wear, and makes it easier to keep them clean in the future. But even if for years you do relatively little to keep your vehicle clean, a thorough detailing when you want it to look its best—possibly to sell it—will make it look surprisingly good.

It used to be that the main reason cars got junked was rust—in the structural and undercarriage components and in the exterior panels. But car manufacturers have made great progress in building corrosion protection into vehicles—by galvanizing steel components, using better paints and sealants, employing plastics and other non-corroding materials, and making design changes. Today, you can expect body panels and other exterior components to outlast the engine and other systems.

Neither the International Carwash Association, which represents professional carwashers and detailers, nor the manufacturers of carwash and detailing products can present hard evidence that detailing—or even washing—does much to prevent corrosion or adds much to the life of the finish. When you sell your car, the slightly higher price you’ll receive as a result of regular washing and periodic detailing is not likely to offset the cost of even two or three detailing jobs.

On the other hand, a good detailing job can do even more for the appearance of your vehicle than a new wardrobe and new haircut can do for your looks. And there is one other benefit from detailing: Cleaning your car’s carpet and upholstery can reduce or eliminate odors and, by removing dust and mites, have as favorable an effect for allergy sufferers as cleaning the carpets in your home.

If you want a professional detail, our ratings of shops for quality and price should help you find the best.