Why do cars need polishing?
Cars need polishing because although bright and shiny when new, the paintwork is actually rather soft and prone to collecting scratches from washing. It can also suffer from environmental contamination such as industrial or natural fallout, and it can become oxidized. All these issues can be worse if the car wasn't given quality paint protection from the beginning in the form of regular waxing or a semi-permanent coating such as a ceramic or polymer coating.
Without a good and regular washing routine, many of these problems can be made worse. Many of the problems which make your car's paintwork look old are introduced by cheap car washes. Both automated car washes and hand car washes can have their problems and do damage to your car.
Having your car professionally polished can greatly improve the appearance of your paintwork, or even make it look like new.