“The term “ground” refers to a connection to the earth, which acts as a reservoir of charge. A ground wire provides a conducting path to the earth which is independent of the normal current-carrying path in an electrical appliance”.
As a practical matter in household electric circuits, it is connected to the electrical neutral at the service panel to guarantee a low enough resistance path to trip the circuit breaker in case of an electrical fault.
Attached to the case of an appliance, it holds the voltage of the case at ground potential (usually taken as the zero of voltage). This protects against electric shock. The ground wire and a fuse or breakers are the standard safety devices used with standard electric circuits.
Is the ground wire necessary? An appliance will operate normally without the ground wire because it is not a part of the conducting path, which supplies electricity to the appliance. In fact, if the ground wire is broken or removed, you will normally not be able to tell the difference. But if high voltage has gotten in contact with the case, there may be a shock hazard.
In the absence of the ground wire, shock hazard conditions will often not cause the breaker to trip unless the circuit has a ground fault interrupter in it. Part of the role of the ground wire is to force the breaker to trip by supplying a path to ground if a “hot” wire comes in contact with the metal case of the appliance.
Source: (hyperphysics, 2014)