You can switch between the enhanced battery life of the Intel graphics while running on laptop battery power and the performance capabilities of the discrete graphics controller while the laptop is plugged into AC power. In other words, you can get best out the onboard graphics solution if you would own a laptop, but the Asus P8Z68-V PRO is a desktop computer.
Some computers have a hardware switch. Some computers have an icon in the Windows taskbar that you can use to switch the graphics controller. Some computers automatically switch to the Intel graphics if the computer changes to battery power, or it can switch to the discrete graphics if the computer is plugged into AC power. Some computers use third party software to do the switch (LucidLogix software)
I hope it helps