Since we are on tank water (water collected from the copious amount NZ winter rain) it's really great to have some water you can literally throw around. If it has been raining alot, and you don't have time to bucket it out onto your garden (or there is a hose from the tub, but it is low pressure and slow), then you can just choose to waste it and switch the hose to the drain.
Now that I see how it works right there in front of me, it is very simple. Each load of washing fills the tub about 50%. I can't believe how much water I have used, and wasted, doing all the loads of washing I did on the North Shore, when I wasn't aware at all of the water leaving the washing machine - and how much water is wasted by the entire North Shore! In general, having a limited water supply just makes you aware of water waste all the time, of my wasteful water habits.