[Keppl8R]

1. If you were to take apart an outlet and look at the contact wipers that the prongs slide into, you would find that they have bumps on them. These bumps fit into the holes so that the outlet can grip the plug's prongs more firmly. This detenting prevents the plug from slipping out of the socket due to the weight of the plug and cord. It also improves the contact between the plug and the outlet.

2. Electrical devices can be "factory-sealed" or "locked-out" by the manufacturer or owner using a plastic tie or a small padlock that runs through one or both of the prong holes. Construction projects or industrial safety requirements may require this type of sealing. For example, a manufacturer might apply a plastic band through the hole and attach it to a tag that says, "You must do blah blah blah before plugging in this device." The user cannot plug in the device without removing the tag, so the user is sure to see the instructions.

3. There also is a small savings in raw materials (metal) for the manufacturer of the actual plug prong. Every little bit helps!

It has been reported that really old outlets used captive ball bearings and coil springs for the detent, but today it is done with a bump and springy copper contacts.

https://home.howstuffworks.com/two-flat-prong-plug-holes.htm

[garren]

None of the above.

#1 is ruled out as he takes apart a number of plug receptacles, from “cheap” to “heavy duty” and “commercial grade”, none of them sport the bumps you’re describing. He also points out that not all plugs have holes (using an ikea timer as an example), and observes that most plug-in receptacles don’t offer the kind of resistance one might expect if such bumps existed.

#2 and #3 are closer to the mark.

Apparently the holes are optional according to a standards body called NEMA (https://archive.org/details/NEMA-WD-6-2016/page/n24/mode/1up) and are generally intended for use in the manufacturing process.

“Hole in flat blade is optional, and is intended for manufacturing purposes only.”

See 13:30 in the video for an example.

[yladiz]

I'm not sure if you watched the video, but if I understood your first point, he debunked that by testing it on a wide range of plugs (sure it's anecdotal but I doubt he has such different plugs from other people in the US). Apparently the plugs that actually do have the bumps don't fit in a way that is safe (either they fit accidentally and/or fit too far forward, meaning the plug, with the holes engaged, is partially exposed). The reason plugs don't slip out is because of friction, not because of the holes.

My suspicion was the third one, and I suspect that was the original reason manufacturers did it. It's likely done now because of the machines that make plugs expecting those holes.

[rainbowzootsuit]

I have a number of locking cord ends (outlets) for extension cords that do use the holes to retain the male plug via ball detents. They require you to push a button to release the plug.

https://www.homedepot.com/p/15-Amp-Stay-Plugged-Cord-Replace...

I've never seen anything else similar.

I'm curious on the design of "hospital grade" plugs as that wasn't in his survey and I know that they are designed for higher retention force than commercial or residential applications.