The outer metallic case of the appliance is earthed as it is used to prevent electrocution of people handling the device. Earthing is accomplished by connecting the device to the ground through an earth wire. If an earth wire is provided, it means that the current flowing through it to the ground has a very low resistance path, preventing the individual from receiving a shock.
Why is it necessary to earth metallic appliances?
The metallic body is connected to the earth wire, which provides a low resistance conducting path for the current. Thus, it assures that any leakage of current to the appliances’ metallic body keeps its potential to that of the earth, and the user may not get a severe shock.