Wouldn’t it be nice if you get a new phone, and you install Angry Birds, and you can just continue where you left off?
When a user turns on their Android phone for the first time and goes through the initial setup, they are offered the chance to opt-in to have Google back up their apps and settings to Google’s servers. If they opt in, all of their backed up settings will be restored and previously installed apps will download from the Google Play Store. As a bonus, those apps can also backup their settings to be restored on a new device as well. Of course, the developer has to configure their app to handle this, and not enough do. Anything less than all apps is not enough.