There are a heck of a lot of reasons to bring out old exercises periodically. An exercise can feel (and sound!) quite different after time away from it. Sometimes we overdo a magic pill exercise so much that it isn’t effective anymore. When an exercise becomes automatic, it can potentially help or hinder us. Putting an exercise away and coming back fresh can help us get more juice out of it, and be a better judge of its value.
Or, we change physically or artistically and the exercise doesn’t fit anymore. Good to know; then we might put it away forever. Or, the exercise that helped us to conquer the passaggio 10 years ago no longer “does anything” because the voice already does the thing all the time as a matter of course. Or, an exercise that seemed to be good for us long ago is actually not good for us, in light of what we have learned since then.
Every exercise needs to have a purpose. It can be as simple as “warm up” or as fancy as “large dynamic variation on every note in my range”. If one exercise isn’t helping with the purpose, there are many possibilities for other exercises to accomplish it. Never do an exercise just because it was what you were taught, full stop. Ask your teacher or yourself why. A productive practicer has a personal intention for every exercise, and take time to evaluate its effectiveness.