The grounding electrode limits the potential difference between the grounding conductor and the earth.
The earth's "Voltage" can vary from place to place because of the chemical makeup of the earth in that given spot.
I'm no expert, but this is the way I understand it.
Without the grounding electrode, there could be a potential difference between the equipment metal case and the earth one is standing on.
Sorry, bad grammer, ...upon which one is standing, rather.
