I have Video in motion on my 1990 7 Series. It was as easy as switching two wires going to the head unit
I think the main reason for it is for your passengers - kids, whoever. You certainly shouldn't be watching while driving!
I can also set the speed at which the video playback turns off. So if you'd like 65mph, 100mph, or 180 mph, I can set it to them or anything in between.
Not that anyone should be watching TV in the car at 100+ MPH