Brake rotors can wear out in several ways. The most common reason for replacing brake rotors is because they are "warped". Warped brake rotors is another way of saying that there are uneven deposits on the rotor surface from the brake pads. This can cause the steering wheel to shimmy while braking. Rotors can be machined to remove the deposits but this is usually counterproductive and it's often safer and easier to just replace the rotors. One brake rotors reach their minimum specified thickness they must be replaced.