If you paint a component with a material, all the faces inside that component which has Default material will inherit the components material. A very handy feature.
It means you can model a car, leave the body with Default material, and add all the rest - like tires, lights etc. Then when you place instances of that car component you can paint each instance with a different colour and the car body will reflect that.
I'm guessing this is what you are referring to.
Maybe a sample model, just to be sure we are talking about the same thing?
