Limited editions allow for slot cars to be collectables. It creates a platform where toy cars can be considered investments. You can’t afford to wait for a car to be used hoping that it will become cheaper. There’s a good chance the longer you wait the higher the cars cost will rise.
Both Carrera and Scalextric make limited editions of their cars. Scalextric likes to put their limited edition cars in a specialized box. It’s usually a purple box with a special paper wrapper around it.
Carrera doesn’t really do that, but they build a limited quantity for most of their cars.