Many reasons to have display on

One, you can confirm you have right program loaded. As an example, my Aces High for squadron use is Aces 9. My Program/defaults control for non squadron flying is Aces 10
Also, you can confirm key strokes and what they are for by the display, rather than going back into the program software and checking the print

Much more easy with the display.
Also, you can use the clock for time and set elapsed time of a mission if you are so inclined

Say you are are in one time zone and your squad mate in another, you can set the second clock for that time zone
