Somehow the indoctrinated will claim that this is a feature.
That means that when you restore a backup, the app should be exactly as it was when the backup was taken.
That includes all files stored in the app, accounts remaining logged in, settings exactly as they were.
Some things won't be backed up to save space/time (e.g. if you downloaded a Netflix movie offline, it won't be backed up), but it is almost a perfect replica of how things were before the backup.
Android has support for a similar concept, but apps can opt out.
If the app stores files in a non-standard location, they won't be backed up.
One of the worst parts of setting up a new Pixel device is having to log in to dozens of apps again! It's a pleasant surprise when an app used the backup framework properly.