why does one need two GPUs
Switchable graphics, its usually associated with certain high end laptops.
One GPU is run during tasks that don't require a heavy graphics card. In this case the Intel HD would be running during web browsing, playing music, writing documents, you get the idea.
While gaming the computer (usually automatically) will switch to the Nvidia card.
The reason is for better power management. A laptop would burn more battery power supporting a heavy graphics card all the time, so it switches between one that uses minimal power but has crappier performance.
Downside is they are apparently a bitch to get driver updates for.
