True, it would have no direct functional purpose. It's conceivable that dimming the lights and such serves as a psychological reminder to the crew that they are in Sekrit Mission Mode and should be cautious about any unnecessary expenditures of energy, and to lock down otherwise ordinary procedures that might give their positions away, and to not touch buttons on the edge of their consoles when they're trying to stand up again, though.
I think that's all a bit silly, but subtle things can reinforce attitudes and if you're running in an environment where mistakes are serious problems, then little things that discourage them are probably more valuable.