Dowsing rods are an excuse to trust common sense; that dip in the land where plants are greener is probably a good place for a well. But digging wells is expensive so people want something more than a guess, but also something cheap.
When I was a kid, a new house was being built downhill from my parents and they dowsed out a "good" place for a well. They were all set to drill before my dad went out and told them they were directly downhill from our septic tank.
What's "downhill from a septic tank"? Isn't a septic tank a container that slowly fills up and is periodically emptied by a truck or something? There is no "flow downhill", is there? I thought the whole reason to have a septic tank is that you have no place for the stuff to go.
Unless you're in a location where you absolutely can't leach the liquid waste out into a leach field (like right on a lake), the tank usually just settles the solids and give the liquid some time to mingle with whatever biological processes are happening in the tank.
When the ground doesn't perk naturally, it's common up here to build a mound system where you have a mound of soil that does perk and vegetation (grass) on it to take up the liquid.
A fully closed tank is basically the last resort. We have friends with a house right on a lake, and their tank had to be pumped every three weeks before they had a kid. I can't imagine what the interval is now.
The best brain power a human possess is subconscious-- its all the perception that allows you to run fast around moving people and catch a ball. You can do it but you can't explain how. You can probably train yourself to be good at finding water or reading people but you might be bad at explaining why. Worse-- trying to explain yourself might be forcing you to use the feeble symbol processing power of your brain. So a prop helps you feel your way.
When I was a kid, a new house was being built downhill from my parents and they dowsed out a "good" place for a well. They were all set to drill before my dad went out and told them they were directly downhill from our septic tank.