If it is really only to be used when iCloud Photos is turned on, why not do the hashing on Apple's servers when the photos are uploaded to the service? It is my understanding that this kind of hash-checking of files is a pretty standard thing on major cloud storage services.

I can only think of two reasons for this:

- for a future expansion into files that are not uploaded, even with cloud services turned off

- Apple doesn't want to foot the energy costs of doing it

Finally, it should be noted that this will also apply to macOS Monterey, not just iOS/iPadOS.