A fair question.
I have moved to a new property twice in the last ten years and each time have had a great deal of trouble finding my below ground level solenoid valves even when provided with a rough map of their locations. Mostly installed at the same level as the underground piping they quickly become buried by soil, fallen leaves etc, and or overgrown by grass. The situation is further complicated by the fact that I am now in my sixties and my memory is not as good as it was.
I believe that commercial landscape gardeners use such a device to locate the existing solenoid valves of their clients.