Not sure why you would use it only if you had a tang. Some people use them some don't. Basically its a grounded probe that sits in the water and is attached to a grounded source. The idea is to prevent electrical current in the tank from shorts in power heads, heaters, etc.
My take on it is that your better off without it because if you have a short in the tank and the tank is not grounded, there is no place for the current to go and no electrical flow. With the grounding probe you give the short a place to travel to through the tank to the probe causing an electrical field.
But thats my take, again, some people use them, some don't.
_Scott