This was tested on a ThinkPad P70 laptop with an Intel integrated graphics and an NVIDIA GPU:
lspci | egrep 'VGA|3D' 00:02.0 VGA compatible controller: Intel Corporation Device 191b (rev 06) 01:00.0 VGA compatible controller: NVIDIA Corporation GM204GLM [Quadro M3000M] (rev a1)
A reason to use the integrated graphics for display is if installing the NVIDIA drivers causes the display to stop working properly.
In my case, Ubuntu would get stuck in a login loop after installing the NVIDIA drivers.
This happened regardless if I installed the drivers from the "Additional Drivers" tab in "System Settings" or the
ppa:graphics-drivers/ppa in the command-line.