Think of it this way, gravity is the most efficient way to move millions of acre feet of water, you want this as high as possible so you don’t have to pump it.
Hydro Dams work on the same theory, gravity. The potential energy stored up behind dams.
How you use that energy matters, the most efficient way is to use it to move the water itself.
In Most of the places with limited water, they need massive reserves of water, this includes water coming over mountain ranges.