I have seen the Electric, Plug Holder 4.5V/12V on bricklink a few times and have often wondered what it is for.
According to Bricklink, it was only ever produced in Light Gray and appeared in 3 sets between 1987 and 1992.
Two of the sets were just Service type sets containing cables and small elements with no instructions or suggestion of usage of the Electric, Plug Holder 4.5V/12V itself.
However, it also appeared in set 9700 Technic Control Center. Unfortunately neither Peeron nor LetsBuildItAgain have a listing for the instructions. So I can't see the step where any of the 5 that were included with this set are used.