Here’s another gaming use: you can have your game on one monitor and web camera feed on another.
Oh, and one exception to the statement that games mostly aren’t designed for multiple monitors: The Nintendo DS portable system has two monitors, and most games for it are designed to use both of them. There, the key is that when you’re developing for that system, you know you’re going to have two, and can count on it. By contrast, if you design a two-monitor PC game, you still need to be able to play the game on just 1, because that’s what most people have.
Just one for me - and I barely have room on my workstation tabletop for the one I have.