Qualifying this, when you run treated water down the drain without using it for some purpose you are essentially "wasting" the local supply of clean, treated, safe water and the efforts, energy and money invested in delivering the water to you in that condition.
Most places on earth have a limited supply of fresh, safe and clean/ready for consumption water available. Especially surface water that you could scoop up with your hands. For example, if you live in Chicago you probably would not want to regularly consume water from Lake Michigan (fresh water which is contaminated with sewage, chemicals, pollutants, etc.)
Water that goes down the drain rarely ends up back in the same place it came from. The clean water you send down the drain in a municipal system becomes mixed with all sorts of other contaminants and sewage. Sewage treatment systems rarely discharge water that is as clean as tap water.
A lot of water used for irrigating crops and lawns evaporates. It might condense into clouds but it will be a long time (if ever) until that water falls as rain back into the source where you got it. Plus odds are that it will fall as rain and mix with other contaminated water (contaminated with harmful viruses, heavy metals, protozoa, bacteria, salt, silt, etc.). Water used for irrigation which seeps into the earth carries pesticides, herbicides and other nasty things with it.
Even if you have a well, it takes energy to pump that water to the surface and your home(usually electricity) so you would be wasting that energy if you let the faucet run. And water flowing into your septic tank/leach field may not eventually drain back into/recharge the source for your well.