What is the function of earth wire in electrical instruments? Why is it necessary to earth the metallic electric appliances?

Hint
Generally, in all types of circuits the negative wire is connected to the ground or earth because the ground wire will not carry any electricity. In any circuit, there are a minimum two wires which are used to function the electric component, one wire is power wire and the other wire is earth wire.

Complete step by step answer
The function of the earth wire in electrical instruments:
In every circuit, one wire is connected to the earth, the main function of the earth wire is to protect the sudden damage of the electrical appliances or the electrical instruments due to the sudden voltage increase or the leakage of the current. Earth wire is also used for the safety measures. Accidently if any current is leaked, earth wire helps to ground the leakage of current.
Necessity of the earth wire in metallic electric appliances:
The metallic body of the electrical instruments will be connected to the earth or ground by means of the earth wire or ground wire, if there will be the leakage of the current the current is sent to the ground by means of earth wire. When the earth wire is connected to the metallic body, it gives the low conducting path to the current. Thus, the earth wire keeps the potential of the leaked current to the earth. So, that the user of the metallic electric component may not get the shock.

Note
The earthing is the process of connecting the negative wire to the earth or ground. The earthing of the metallic instrument is necessary to stop the electric shock to the humans. The excess leakage of the current will damage the electric instrument or the user, so the earthing is done in electric circuits.