Over time scratches on a vehicles paint multiply and leave a dull finish. This can be avoided on a new car if you use the right products and technique but used cars can suffer from heavy swirling/scratches. Most people or car washes will use a circular motion, so over time these scratches build up and create swirls. When buying a used car swirls and scratches tend to be a given as most owners don't tend to know the correct technique, so having the car detailed can take you back to how the car would look when new (and untouched by the dealership) and then with the correct products and technique you can keep it in that new condition. The owner of this Nissan 350Z had just this thought when he booked in to get his car as good as possible.
Below is an example of the swirl marks:
Now the challenge with this is to correct these scratches but in the safest way, so removing just the right amount of clearcoat, so the marks aren't visible but you still have lots and lots of clearcoat. The problem with this Nissan was very low paint depth readings. So care is taken to remove the scratches but leave as much paint as possible.