The Probert concept was for a grand officer lounge:
(as seen in the final film here when Spock’s shuttle approaches):
They had to scrap this as they were behind on budget and time.
Both DC changes (first adding the nacelles and then adding this backdrop) are at attempt to bring this to the film.
Both attempts unfortunately look very poor and they should have just avoided doing so (the added nacelles screw with the visual weight of the frame, and the attempt to blow out the wall looks amateurish). This video is a decent illustration of what they are conceptually going for, though:
https://www.youtube.com/watch?v=PU1-crmRpCc