My desktop is split between two monitors; the left monitor is for regular work and is usually covered up by full-screen (or close to it) applications. The right monitor is full of a cascade of windows, so each window is selectable from their bottom left corners.
Apart from that, the background is just used to show pictures. It's surprisingly awkward with multiple monitors: I had to write a program that composed the pictures I wanted to display, then set that new picture as a tiled background. If you don't do that correctly, and you have multiple monitors with different resolutions, you end up with either the same image on all monitors, or clipped / stretched / distorted versions on all but the "home" monitor (the one that owns (0,0)).
Apart from that, the background is just used to show pictures. It's surprisingly awkward with multiple monitors: I had to write a program that composed the pictures I wanted to display, then set that new picture as a tiled background. If you don't do that correctly, and you have multiple monitors with different resolutions, you end up with either the same image on all monitors, or clipped / stretched / distorted versions on all but the "home" monitor (the one that owns (0,0)).