Answer: When brake pads are compressed against the rotor, the friction created stops the vehicle. This friction eventually wears not only the brake pads, but also the rotor. Rotors develop wear patterns from the brake pads over time. When a worn brake pad is replaced, rotors need to be machined smooth to remove the wear patterns. A rotor can be machined smooth only so many times before it is worn too much. Each rotor must have a certain width, which is inscribed on the rotor, and if the rotor is too worn, it must be replaced. Over time, rotors can also develop heat cracks and other damage.