It turns off the front lights. They will cause a glow in your night footage.
I learned this lesson the hard way with my Phantom 2 Zenmuse. Back then you had to tape them off as there was not even an app for that quad.
Watch this movie to see what happens when the front 2 lights are illuminated... (*It used to have cool music till YT cracked down on me)
Those front lights can also project onto near objects in front of the camera, for instance if you use the X-Star as a walk-through camera inside a building and take advantage of the smooth gimbal and 4K for the video.