Apple introduced fast user switching years ago to allow easier shared use of a single Mac by multiple people. After logging into an account and switching away from it, all the activities that were underway continue on their merry progress even while another user has logged in or switched to the foreground.
This includes iCloud Drive, Dropbox, and other sync apps that carry out their operations in the background during a normal session. This can be useful if multiple people rely on synchronized files, preventing them from having to log in and wait for updates to occur.
Apple assures its third-party developers in fast user switching documentation that background accounts have no keyboard or mouse input, and apps that check on screen activity will receive a response as if the screen were asleep. That is a sort of guarantee that any app or service that’s running will reliably keep running.
