The Nvidia GPU is a discrete graphic controller. The Intel GPU is integrated into the CPU, hence both show in the system tray and Device Manager. The Discrete GPU in the 7580 is a hybrid video, (the "discrete" GPU boosts the Intel GPU, but CANNOT run independently of it). Thanks to a forum post by ejn63 for that information.
I just got a new Inspiron 7580 with NVIDIA GE Force MX150 graphics, but it also has Intel Graphics software running in the system tray.  Why both?
Both Intel and NVIDIA graphics software on Inspiron 7580.