Who needs this app?
It depends on user preferences. Personally, I don't like when side monitors are bright. They distract me. A ctually that was my only problem with working on multiple monitors. So the perfect solution for people like me is to dim them a little bit when they're not used. Doing that manually (physical button) is bad idea. Turning them off is even worse. ScreenFocus follows my cursor position and does the job for me.