Why do electrical plugs have a hole in each prong?

Question

Except for the ground wire in a three-prong plug, virtually all electrical devices that plug into a standard 120v AC outlet have a small hole in each prong. What is the purpose of this?

Answer 1

"Within the outlet, there are 2 plates with dull indentations that press into the holes and help hold in the cord, so it doesn't fall out as easily.

Also, some special outlets allow you to lock the cord into the socket, so it can't be removed, by putting rods through the holes. A few bars and pubs have it, so you can't unplug the vending machine."

Source: http://everything2.com/index.pl?node_id=...