Why would a city put its water supply on top of a mountain?
The reason has to do with gravitational potential energy (GPE). If that water was needed quickly, let's say to put out a big city fire, all the town would need to do is open the water gates. Due to the force of gravity, the water would naturally stream down the mountain to earth's surface.
Directed properly, the water would put out the fire all by itself and the town would be saved.