Does using brightness dimmer (screen filter) really save battery?
When you decrease the brightness of a light, it normally means less power is consumed. Therefore, having a lower screen brightness than your default would suggest battery use is reduced further than normal.

As far as I can guess, the screen brightness will just be a command in the android system that changes power flow through the screen LED's. The phone manufacturers likely just set their brightness slider's lowest value to whatever arbitrary value they expect a standard user would require. The app's that you are using just influences the same function, but drops the power lower. 

e.g. 
Samsung use a minimum screen brightness of 30% power, but your app will drop to 10%

I don't know the specifics of such tool.