There are a number of reasons why a beekeeper is likely to requeen a colony including the following:-
The temper of the colony is unacceptable.
The characteristics are not what the beekeeper requires. Bee genetics are an important part of beekeeping.
The queen is failing or showing signs of failure.
The queen is getting “old”. Regular requeening simply on age is often advised to keep a colony productive or to reduce the chance of swarming. This is only relevant to prolific or swarmy bees and something I don’t do because with the kind of bees I keep regular requeening isn’t necessary.
A colony is showing susceptibility to disease.
Replace a queen in a queen mating hive.
I think that beekeepers need to be careful when requeening colonies. I have seen some very good queens culled for poor reasons. These can be simple things like not understanding that colonies with non-prolific queens can be more productive than those that fill the hive with bees, or the brood pattern looks bad when there are other reasons for it. I have found that before a queen is culled the beekeeper needs to be absolutely certain that is the best choice.