Why is polishing a car important?

In a general sense, having a car which is polished makes it look like new. Dull and dead paintwork, wash marks and scratches are what makes a car look old.

Cars these days tend to be very mechanically reliable and the engine and running gear can last a long time, so this is less of a concern in valuing a car. Much of the car's value is its cosmetic appearance. So a car which is polished and shiny, and looks newer, will be in more demand and fetch high prices at sale.