I think it's a personal preference. I only find my second monitor useful when I'm working in programs that have a lot of dialog boxes that I keep open, or if I'm following a tutorial I can have the video on one screen and work on the other. Basically if you find your single monitor filled up and congested, you might look into a second monitor.
For gaming I'm not sure if you could take advantage of it, I don't how many games support multiple monitors.