Can someone explain to me how to use a water level to level for a pool?

I understand the concept of using a WL, but I'm not quite understanding how it'll make leveling for a pool easier. It seems to me that I'd have to put the stake in the ground a million times, and measure over and over and over again to make sure things were right.

In my mind, it seems easier to use a 2x4 and a normal level....

Am I missing something totally obvious?