Just stumbled across this on my 08 GMC Savanna with auto DRL.
With the screen display setting on auto, there were many instance the nav screen would go in the night view on an overcase day making the black background very difficult to see. By clicking on the day/nite icon on the nav screen you can have a choice of having the background go black or just slightly dimmer than the day setting. This make the nav screen display much easier to read when your vehicle light system is on automatic.
The photos below show the difference between these three available settings.