2 monitors are useful for a few things.
Some people like them for gaming, so they can have a game in one and have the internet/facebook/reddit/stream in the other.
2 monitors are very useful for video editing, so that you can see a large image in one and the timeline/color corrector/etc on the other side.
Also can be used if you have a lot of docs/spreadsheets/ whatnot on one side, and a web browser on the other
Its a luxury most people don't need, but it's nice to have.