In the early times, the power plug has a small hole to fix, but now it's different;

In fact, the purpose of the small hole in the prongs of power plugs is of heat dissipation, especially in the joint.

Since the power plug is not a whole, there will be resistance at the joint, which produce heat after using for a long time. That’s to say, if the heat is not dissipated in time, the joint will turn soft. Finally, it will outbreak a fire due to the short circuit.