Its basically an extended desktop, mainly for people who need a bigger area to work, and are limited by 1 monitor. And example would be having multiple spreadsheets open at once, or browsing the web and working with photoshop.
Only some games are capable of using the extended desktop feature, most other games simply clone the image on both monitors. But as mrbagrat said, this does cause more strain on your gpu, but its mainly caused my having such a high resolution.
All in all, extended desktops are useful if you need alot of desktop room, other then that it gets a little annoying having such a wide monitor with a gap in the middle.