If they buy an iCloud storage plan, iCloud will be their “backup”. If the device fails, their data lives on. Get a new device and all their data is “restored”.

If they buy an iCloud storage plan, iCloud will be their “backup”. If the device fails, their data lives on. Get a new device and all their data is “restored”. Of course it’s a sync service, not a backup service, so you can’t role back. For the average user though I guess this doesn’t matter. (macOS takes local snapshots if unused disk space permits. So you could revert back some files if the change is recent.)