Drawing on the screen allows you to sketch anywhere (in other applications or games) on your device's screen while it's running. Regardless of your current on-screen activity, you can easily toggle the drawing mode on or off with a notification.
Reversing screen drawing is typically done to create a screenshot: first, draw on the screen, then take a screenshot, and finally, share it.
Screen drawing assists users who are recording videos (or screen capturing) when they want to emphasize something.