A bicycle disc brake rotor does not last forever. The pads gradually wear down its outer braking track even while the inner portion may still look nearly untouched. It is more useful to compare the thickness of the braking surface with a less-used part of the rotor and check the minimum specified for that particular model than to judge the disc by its overall appearance.
For example, some Shimano rotors begin at 1.8 mm and are recommended for replacement at 1.5 mm, while a Hope example starting at the same thickness has a different 1.4 mm limit. These numbers are not a universal rule for every disc. Even if the rotor still appears straight, a pronounced groove or step at the edge of the braking track is a reason to measure it and compare the result with the maker’s specification.
Wear is different from a bend: lateral wobble may cause rubbing, but straightening cannot restore a braking track that has become too thin. When a rotor reaches its replacement limit, inspect the pads as well because the two components work together. A regular check is particularly useful after wet or dirty rides, when changes at the braking surface are easy to overlook.