What are the reasons for having multiple monitors? Seriously... (besides having documentation, guides, etc easily available on the 'other' monitor.
Having web browser, teamspeak, steam, etc. open on the second is the main reason. Of course this also requires that games support fullwindow/borderless mode, to be able to swap without minimizing.
It makes a much bigger difference for productivity than games, though. I run 4 monitors (if you include the laptop itself) at work, which lets me have outlook, multiple instances of visual studio, documents, etc., all on screen at once.