No one needs dual monitor. Not even high end pro's doing stuff like CAD or a graphics pro working on multiple apps at the same time. For instance, someone working in Photoshop at the same time as Painter and having a page layout program open. Now, that's not to say it doesn't help their productivity immensely to have multiple monitor support.
That said, dual monitor is something that grows on you. It's one of those things where if you've never experienced it you will never miss it. However for those who have gone the multiple monitor route then you know that it simply increases the user experience. It's great being able to play a game but at the same time browse a web page without having to switch foreground apps. You can see all the happenings in the game while browsing at the same time because all it takes is a glance to see if your attention is needed on the game. Sure, you can do the same thing on a single monitor but sometimes things happen on the game that needs your attention right away but you won't realize it cause your web browser is the foreground app. Another use is web page development. You can have all your editing tools on one monitor with a web browser in the other that you can instantly update to see the changes without having to switch foreground apps since both the editing app and the browser app are effectively in the foreground on their respective monitors. One thing I do sometimes is watch recorded shows or a DVD on one monitor while browsing the web on the other during quiet scenes. Not everyone needs dual monitors but it's nice to have.