Why on earth would you want more than one screen!
for the same reason you have your current one.
so you can do things on the computer
the more space you have, the faster and more efficiently you can do things on your computer. mainly because it saves you from having to constantly switch back and forth between numerous programs. it really helps with things like computer/web programming, video/sound editing, 3d modelling, graphics designing, etc. having multiple screens helps pretty much any time you might need to have more then one program open.
it also aids in multitasking. can play a game in one screen, while having a a list of cheats or the cheathappens trainer on the other screen for easy reference. watch a movie in one screen while typing on the other screen.