Whether it's after a car has sat in an airport parking lot for a week, or from being driven in a corrosive environment, rust on the friction surface of the rotor can cause noises and vibrations.
Using the Pro-Cut process on rusty or unevenly worn rotors also has the added benefit of improving their appearance. With rotors that can be seen easily seen through performance wheels, this can be important to many customers that take pride in their high end or performance vehicle.