Why are there two holes in the plug?

Some socket contact point will have a protruding bag, just and plug on the hole anastomosis, play a fixed plug role. The hole in the plug can also be used to lock it and prevent the child from getting electrocuted.

[cowwcmk]

The real answer is that the holes give the machine a place to hold onto the plug during manufacturing. If you see a plug without holes it is because a different assembly style was used.