What does a ground rod do?
A ground rod simply lets static electricity return to planet Earth. All other electricity could care less - it just wants to go back to the transformer that 'made' it.
A metal water main is sure to be an excellent ground. The only problem is that we use plastic so often, we can't be sure it's all there anymore.
So, we make a ground electrode - be it a plate, a rod, or a Ufer.
What if the utility neutral is lost? Then electricity might very well travel through your water pipes, cross over to your neighbor's house, and return using his utility neutral.
That's why, if there's a shock hazard in a house, you turn the power off at the main, and TEST to see if power is still present. If things are still hot, then power is coming from elsewhere.
