Wasting Water

Can someone please explain something to me? Why do people always tell you not to waste water? When you pour water down the sink, it goes through the sewage system, gets filtered a little bit, and then either sucked back into the water system or dumped into a lake or stream. That water can then be filtered and used as potable water, or it will evaporate and become rain. Either way, the water does not escape the system. The water is not destroyed so that we need to find new sources of fresh water. Am I missing something? It seems we're wasting the electricity of the processing plant, not the water itself.

Also, potable is a stupid word. It just means drinkable. Why don't people just say drinkable! (I see the irony in using potable above and then bitching about it here. I was using it above to prove this point!)