I was confused too until I tried it
Without any kind of software support, a DualHead Vid card will just output the same image onto the 2 monitors. Now with software in the OS, basically the 2 monitors become a giant desktop. Like, I've got a R7500 with a 17" @ 1024x768 and a 13" @ 800x600, so basically, the desktop is now 1824x1368 and it gives me more workspace. Unfortunately u cant like switch between a game and ie window type of thing simply because when you click outside (this is the case with most games) the game's window, it minimized, and defeats the purpose.
In any case, for a person like me who spends most of his time surfing, and working in Word/Excel docs, its a great and very useful feature even with a cheapo setup like mine where the 2nd monitor is a old 13"