MAYBE to "fix" a problem where you have a piece of furniture pushed close to the wall, and there's not enough room for a standard plug to connect into the outlet- so here you can 'swivel' the outlets 90° so you can connect them.
Either way, it's UL-LISTED! Haha.
-Ray
Ya, UL listed from 1955........
Agree with the reason for their existence. Plug in a couple of lights, when the outlet is behind a chest of drawers or a couch.
Back in the day when houses had 100 amp (or less) services.
An old friend of mine had a Hydro-Quebec installed interlock between the dryer and dishwasher. You could only use one at a time.
They now have 3 prong versions of basic ally the same thing.
Chris.