Sometimes it’s nice to turn off your phones backlight but not let it go into sleep mode. By doing this, it lets background events continue without draining your battery by having the screen on. If you’re using a GPS app, for example, it can continue to track you. Another example is using Active sync where the device by default leaves the backlight on. XDA member virgilp created a shortcut for WM to let you press to turn off the screen without putting the device into sleep. There are other apps that can do this but simplicity is a good thing. The app is located here.