A screen filter basically is meant to decrease the brightness of your screen below its minimum system-allowed level, so it would not hurt your eyes when you use your device in the dark. It also saves quite a lot of battery power on devices with AMOLED screens. However, all of the screen filter apps out there provide a static screen filter which just stays on the screen all the time, regardless of whether it's actually needed or not. The problem is that when you find yourself in a very bright environment, where you need the absolute maximum brightness which your display is capable of, a static screen filter would take away from it, so you would have to manually go and turn it off.
