Braking rotors rotate as your vehicle rolls down the road. When you press the brake pedal, the master cylinder sends hydraulic pressure down brake lines to the calipers. This is where a piston (or pistons) is forced out of a bore, pressing against the caliper housing and the brake pad. High performance cars need better brakes than stock cars to slow the car from higher speeds, faster. Increasing the surface area is the best way to do this. The only real restriction of the rotor and caliper assembly, up to a limit on rotor size, is wheel clearance. With 15 inch wheels, the largest rotor you can have has an outer diameter of 11 or 12 inches. Unless you have very low profile calipers, you need 3 inches of clearance at least from the rotor to the inside of the wheel.