Most of the windows managers are optimized around maximizing a window on a single display. So often you would have one screen maximized with one thing, second with another, and even better third. Simple as that.

If you are a game level designer, or builder, scripter, modeler, etc. you need one screen for the game, one for the edito, one for something else - it might be docked out windows from your editor.

Even coding - debuger, terminal, browser, or log viewer, etc.

Then you have people having irregular screen orientations - one screen rotated sideways - maybe for coding, or logs, the other one - for visualizing things.

I was firm unbeliever in more than one screen, until I was finally sold...