2012-2013 I wrote a parental control app for Android that "locked" down the device so you could give it to your kid and it would only run what you wanted it to run for however long you wanted, like "30 mins of games per day". To do this properly, you needed a task manager, ability to kill other apps, guard against being uninstalled, etc. This was not possible with the permissions that were available by being an app on Google Play, it was possible only if your APK was signed by Google or the carrier with system-level permissions.
Nevertheless, I cobbled together a collection of tricks that managed to do it well enough - my app was a home launcher and when it detected a game running past allowed time, it would "kill" the app by pressing the home button, thus brining itself to the front. It had to take over the status bar to prevent you from swiping down to access settings, etc.
Anyway, I relied on APKTool a lot to disassemble other apps which were doing things that I needed (like drawing over the status bar), because they had already figured out how to use these undocumented APIs. I could not have done it without APKTool.