Google's backup service: what is it for?
I think you misunderstand what Google backup is and does, and it varies by Android version and app... I know this can be confusing but prior to Lollipop the only information backed up was system info (wallpaper, WiFi passcodes, etc) and Google info like contacts, calendar, notes, and stuff. Beginning with Lollipop, the installed apps (not their data) are backed up and can be restored from a list on a compatible Android install. In Marshmallow, application configs (not data) that are designed for MM are backed up. Many vendors/carriers have their own backup tools as well.