They are safety sockets, to be used with safety plugs, like the one posted above by Richard W.
I see them used mostly in German equipment, and I believe the reason is to have all cables in the lab with very high insulation (they are rated at 1000 V), with all the plugs protected against an accidental contact (no naked bananas).
I think I've read somewhere that this is the only connector allowed by some safety standard, but now I could not retrieve any information.
It is not an economic reason, they are more expensive than standard binding posts.
The attached photos show a panel socket and a cable plug.