Android has a relatively unpredictable habit of killing any
Activity that is not immediately visible to the user to reclaim memory. Because of this we need a way to test how our app behaves when our Activities are killed. 'Don't keep activities' forces Android to kill background activities (as though it were in a low memory environment).
From the Android Developer Documentation:
Tells the system to destroy an activity as soon as it is stopped (as if Android had to reclaim memory) This is very useful for testing the
onCreate(android.os.Bundle)code path, which would otherwise be difficult to force. Choosing this option will probably reveal a number of problems in your application due to not saving state. For more information about saving an activity's state, see the Activities document.
Don't keep activities is in the Developer options settings of your device.
Consider enabling this to test that your app saves state properly.