Why Earth a Metal Case?

Many electrical appliances have metal cases. Metal cases are dangerous. If the live wire should come into contact with the case, then anyone touching the case will receive an electric shock. To overcome this possibility, the Earth wire of any appliance with a metal case is connected directly to the case.

If then the live wire comes into contact with the case, a live wire - case - earth wire circuit will be created with a low resistance. A large current will flow and the fuse will blow.
Of course then the appliance, case and all be be completely off, and no one will be electrocuted.