Same for me. Genuinely doesn’t understand what’s the benefits for anyone. Maybe trying to get back "lost" hours when a teacher is sick? I think kids are better served with a stable, secure schedule than one that try to absolutely fit all the modalities decided for that school year.

What can I say, my school worked differently and this was a common occurrence. I do believe they tried to minimise room changes or hour changes as much as possible, but you have to do something if the only relevant substitutes would already be teaching another class at the same time.

In my experience, substitute teachers were generally not skilled in a particular subject. There were there to be an adult in the room. They typically had a lesson plan left by the regular teacher, which almost always involved just reading or working problems on your own and did not depend on the sub being able to actually teach the subject.

If you can't find enough substitutes, pay them more instead of spending money on apps to rearrange schedules.