Why resurface discs?

Brake discs should be re-surfaced when:
How it works:

The Pro-Cut lathe works on the vehicle, by attaching itself directly to the hub.
The technology includes an automatic, electronically-controlled, gyroscope system which measures run-out in the hub and corrects for this to an accuracy of better than 0.05mm.

The Pro-Cut on-car brake lathe re-faces discs in situ and is:

Machining discs with Pro-Cut delivers improved braking performance, eliminates brake vibration and excessive noises, and removes rust and corrosion.