The appeal to me of Gran Turismo is that I can drive my car. I don't care for some super rare million dollar UFO looking car. I want to drive modified versions of cars I can actually acquire in real life. Cannot do that with most other games. But I mostly like the early part of the game where you don't have the cash to completely upgrade the car the moment you buy it. That being said forget GT on PC I want Forza. I just cannot bring myself to buy an xbox for one game.
Similar thing for me, but with Forza. I don't like driving race cars in Forza, I like driving muscle cars or other "normal" cars.
One of my favourite Forza cars is the Renault 5 Turbo - tiny little car with a 1.4 turbo engine, fun as hell to drive.
Forza is also good for "what if" questions. What would happen if I took a VW Golf Mk1 and made it as fast as possible?
Or, can a 1970 Corvette ZR1 beat a modern Corvette (the answer is yes, if upgraded properly).
I'll never own either a Renault 5 Turbo or a 1970 Corvette ZR1, but I still much prefer them to an Audi R18 TDI. You can't really improve on the R18 TDI, its already as good as it is going to get.