Water can be expensive, right?
Well, kind of. Water is actually free. You have a lake or a stream or ocean near by and you can basically take water from it at zero cost.
What costs money is moving and cleaning water.
A local water supply is often pumped, and is often treated to make sure its clean enough to use. Then the waste water is usually pumped through a waste water treatment plant. And all this water cleaning uses the magic of chemistry and filtering, which requires a lot of energy.
Every time you run a shower or flush a toilet takes energy, because ultimately that water needs to be moved and cleaned. Chemicals need to be mined and processed to perform the treatment.
In short, water-related costs are mostly energy costs. Using water uses energy, and saving water saves energy.