[bpowah]

Now would somebody please tell me what the little holes are for in the end of the plug prongs!? http://www.worldofstock.com/slides/BEN2097.jpg

[pndmnm]

Many sockets have little bent-wire brackets that pop into those holes when the plug is fully inserted in order to help hold it in place.

[ars]

http://www.straightdope.com/columns/read/579/why-are-there-h...

[sc00ter]

A tangential use (but not the original design intent) is to put a padlock on the plug to prevent it being plugged for safety reasons. (Think power tools, high temperature devices etc).