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. ... It also improves the contact between the plug and the outlet.

What are the holes in prongs for?

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.

Why do plugs have 3 holes?

A three prong plug is designed so that electricity can be safely supplied to electrical appliances. The third prong grounds the electricity to protect anyone who uses the metal-encased appliance from electric shock.