system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does one "waste water" when using it in your home. Doesn't it just get added directly back to the water supply?

Note: NOT in regards to agriculture or outdoor water usage.

I just saw a commercial that said something to the effect of not turning off water while brushing wastes like 4 gallons of water. I don't understand this, because according to my admittedly limited understanding of how our water supply works, the water just goes back into the city water supply after its treated.
gpt
"Wasting water" really means "wasting energy". Not every place on the planet has a nearly unlimited water source (like Washington and Idaho). In places like California, New Mexico, or Arizona, where clean water is scarce, using more water than you need to means that they have to ship more water from other areas and spend a great deal of money and energy to ensure everyone has the water they need. 

In those places that are totally flush with clean water, like the aforementioned pacific northwest USA, you really couldn't waste enough water to even be a bother. In places like those drought areas, wasting water means higher water costs for everyone, and higher costs for the city.