[broadgage]

I allways understood that the hole was to permit of visual inspection of the earth connection.
It was too small for a thorough inspection, but at least if the earth wire had completly pulled out this would be obvious.

Amateurs when connecting 3 core flex to a plug tend to cut all three cores to the same length, rather than leaving the earth longer.
Any excesive strain or loose cord grip would then leave the earth wire as the most vulnerable to pulling out.