Ground wires act as a safety feature, providing a connection to the earth that can neutralize an electrical charge. If a faulty current occurs, the ground wire will trip the circuit breaker and prevent an electric shock.
Three wires are attached to a circuit, two of which are hot wires. Since hot wires actually provide the currents of electricity, the grounding wire is not necessary for the appliance to work. The U.S. National Electric Code requires ground wires and specifies they be tied back to the neutral line at the service panel in a process known as bonding. Bonding greatly increases the safety of the appliance.