This is a traditional feature in Pro photo applications like Adobe LightRoom and possibly Apple Aperture.
It's very useful for photographs, I don't know if it's equally useful for other types of data.
For programming for example I much prefer adapting the background of the window with the code source to an eye-friendly color that I vary during the day. But you cannot do that with all applications.
For everyday use of text-based applications, especially on Mac OS X, where there's lots of white window with small black text (e.g right now I'm writing on MarsEdit, with NetNewsWire in the background), dimming the background application could appease the eye from all of this flashy and distracting white things in the background.
I think I may give a go and try this software.
Enhance Your Mac's Visibility With This Cool Application -It's FREE-: "It's AWESOME. YOu must see screenshoot before you decide. It permits one to dim the background of the foremost application thereby enhancing its visibility. You can switch it on or off very quickly from the menu bar, and set the level of dimness.'"