Screen Tester - If you are interested in the health of the screen of your device, here is the application you were waiting.
You can check if your screen has flaws and if the touch screen works perfectly.
Finally you can clarify your doubts, so if you see strange defects in the image, you'll know if it's the screen or otherwise.
Very useful for testing new devices and its warranty concerning screen defects.
Also, you can check regularly your screen to know its exact health on each time.