Gran Turismo is not just about pure numbers and how fast can my car go, it is also about stellar graphics, detail, and art. It is (from an artist's standpoint, computer or traditional) pretty incredible, and i think alot of people tend to forget about this. Personally, i was bummed when i got GT2 and found that the cars in that game don't lose their shine--making the car wash a waste of memory. I'm glad it's back and working in gT3.
The car wash is in the game simply cuz Polyphony COULD put it in there. It's graphical genius. No, it isn't absolutely necessary, it has no function other than making a dull car shiny again, but that's what seperates GT from many other games--GT is not all about speed and winning.