Earthing is required for equipment that is not double-insulated.

A conductive path to earth that is designed to protect persons from electrical shock by shunting away any dangerous currents that might occur due to malfunction or accident.

Electricity by nature seeks a pathway to the ground and always takes one with the least resistance. If the grounding system of an installation is weak or faulty, either due to wrong or poor connections or a circuit fault, the electricity will try to find the easiest path to the ground. This can be a person touching the exposed metallic part of the object, or the nearest object. The path provided by the safety ground connection to the ground mass is therefore designed to have the least resistance so as to allow the fault electricity to flow easily.

The safety ground provides electricity with a pathway from the source to the circuit and then back to the ground. Ground rods are usually used at the utility company’s power entry point into the premises so as to connect the circuit to the ground.

The GND or circuit 0V is sometimes connected to the chassis (usually metal). This is done so that the chassis acts as a large RF/EM shield.

Now some equipment has the above as well as earthing on the chassis to prevent electrical shock should something short to the chassis (power transformer for example). But this is not ideal in certain applications such as audio where it can lead to "ground loops" and hum getting injected into the audio. The solution is to isolate the system GND from the earthed chassis via a "ground lift" - this allows RF/EM shielding as well as isolates the 0V point from earth at all frequencies of interest and effectively breaks the ground loop.