Some (most?) LCD panels have an option to display the resolution that your computer supplies in their native pitch and just be blank in the parts not covered. While the image won't cover the whole screen under these circumstances, this option usually yields better image quality than whatever unnatural stretching and interpolating the monitor does to put the non-native resolution on the whole screen.

This can also come in useful on an older failing screen. I have a beautiful IBM 18.1" LCD panel from about 2001 which has some kind of clouding/dimming/cruft around the edges. But if I run it in 1152 X (mumble) and tell it to just use its native pitch, then I get a fine image with a small black border and I'm not sacrificing much by not having 1280 X 1024.