Because it's part of a telephone switchboard, where those plugs originated. :)

My guess is it's the ringer from a small exchange, and the reason for three sockets is so the operator could call multiple users at the same time (e.g. for conference calls.)

My suspicion is that it predates WW2 by a number of years, it may even be from before WW1. (I need to find some more books: Instructions in Army Telegraphy & Telephony (the earlier edition), and "Technical Equipment of the U.S. Signal Corps". I will have a further rummage over the weekend.)

Best regards,
Chris.