You don't usually need to do disc work for every brake pad change, but when servicing the brakes, always make sure the disc is within the acceptable range. Once the disc surface shows deep lines, is warped or leads to undesirable brake noises or vibrations, it could be refaced or replaced, since these on-car lathes precisely align the disc with the car's hub and bearing. Each disc has markings showing the thinnest thickness allowed, recorded on an area left uncut. The disc should not be machined below this particular thickness. You should clean the surface of the brake disc face and make sure hub adapters are undamaged and free of rust before you install anything. Kitidleitit as the manufacturer tells you and cut both sides of every disc at the same time to avoid getting an uneven result. Disc turning equipment used during refacing must be handled very carefully to keep the TIR and thickness variation within the required limits.