I have a theory that water filters help save water.  Maybe it's psychological on my part but when we had (yes had - it broke) a water filter, I reckon we were more careful with the water we poured out of it; we treated the water like it was more precious.  Now that the water filter has gone (soon to be replaced) I feel as though we're a little more careless with the water we use from the tap for drinking purposes.  Water from the tap comes out faster and dirtier and perhaps that's why we treat it with less respect than the water from a water filter.  Just a theory though!