When an electrical appliance of high wattage like pressing iron or microwave oven does not have any flaw or defect the current passes through two smaller pins only. No current passes through the (long) third pin and the third wire connected with it. Meanwhile one end of the wire connecting the third pin remains connected with the inner surface of the appliance. Similarly, one end of the wire from the socket is connected with electricity conducting metallic plate which is buried in the Earth. This type of permanent electric connection is commonly known as Earthing.
If a live wire in the pressing iron becomes loose and touches the inner surface then electric current starts flowing in the metallic body and if a finger touches it then one gets a powerful electric shock! Human being’s body is a good conductor of electricity, so electric current which must complete circuit by entering earth passes through the body on its way! However, if a still better conductor is provided, then it is a different matter because the current will take that more convenient medium for its flow. Such better medium for the current to pass through is copper wire connected with the third pin.
There is a reason behind making this pin somewhat larger. As soon as the lower two pins enter the socket current starts flowing to the appliance and if there is some flaw or defect one may get electric shock there and then. But as the third pin is longer, before the current starts flowing to the appliance through the lower pins it is already earthed.