TL;DW: Specification says that the holes are optional, but they are often used to facilitate the manufacture of the plug blades. It defines a position and size for the holes so that they will be out of the way of the actual part of the plug that makes contact

Alec (the person who runs this channel) has a habit of rambling on about all sorts of semi-related historical facts and other such nonsense.

I understand if that is not to your taste. I quite like it.