A common question at brake time: if I am doing pads, do I have to do rotors too? Unlike hardware, rotors are not an automatic replacement with every pad change. Whether they can stay depends on their thickness and condition. Here is how to decide.
Rotors are not automatic
Pads are the primary wear item and get replaced far more often than rotors. A rotor in good shape - above its minimum thickness, smooth, and true - can absolutely be reused with new pads. So no, you do not automatically replace rotors every pad job; you inspect and measure them first - see when to replace rotors.
When rotors must be replaced
Replace the rotors if they are at or below the minimum thickness stamped on them - see rotor minimum thickness and how to measure it - or if they are deeply grooved, cracked, heavily rusted/pitted, or have thickness variation causing a pulsation. A rotor too thin to reuse cannot safely take new pads.
Resurface vs replace
If a rotor is above minimum but has minor scoring, it can sometimes be resurfaced (machined) instead of replaced - though many modern rotors are too thin to machine and it is often cheaper to just replace them; see resurface vs replace. Always do rotors as a pair - see replace both rotors.
Why many people just replace them
Because rotors are increasingly affordable and often too thin to machine, many shops and DIYers simply install new rotors with new pads for a fresh, quiet, warp-free result - and it lets the new pads bed to a clean surface. If your rotors are still healthy, though, quality pads and fresh hardware on the existing rotors is perfectly correct.