Easier multitasking. You can have one thing running on one monitor, and another thing, or several open on the other one. Nice for having monitoring/temp programs on one while you're playing a game, etc. I could go on and on. As for games, you'll pretty much still just be playing it on one monitor. Unless you have an ATI 5000 series card setup with Eyefinity. And a minimum even for that to be worth it you'd want 3+ monitors.
If say you had two, you'd set it up as Extended display, which would just extend one monitor to the other, like one long desktop. Everything wouldn't be split equally from the center bezel though. You would have one thing like your browser on your main, and some other programs running on the other.
Or as a Duplicated display, which would be like having exact copies on both monitors of what you have on one of the monitors at a given time.
If you had two or more monitors, you'd want to have some program like Ultramon, or DisplayFusion to give you some better options to take advantage of the mutli monitors. Like extending the taskbar, etc.
A program like Ultramon would let you split some programs between the screens if you wanted, say like your browser, but it's impractical with two monitors having the middle of your browser behind the monitor bezel.
Eyefinity (with ATI 5000 series cards) can merge all your monitors into one big display for games, but not all games work with it, and has it's own quirks.