Urban Homesteaders know a lot about greywater systems. If you don’t know, household water is usually classified into 3 types:
CLEANWATER (or potable water): the stuff that comes out of your faucet. It has been processed and chlorinated and is safe to drink.
GREYWATER: the water you throw away after it’s been through your bathroom sink, your shower, your bath or your washing machine.
BLACKWATER: the used water from your toilet.
Looking at the diagram, you can see that cleanwater goes to your bathroom sink, bath, shower and washing machine, and becomes greywater. That greywater can then be re-used to irrigate your garden and to flush your toilet: (because what kind of idiots are we to flush our toilets with drinking water?? >_<)
If you live in an area of little rainfall and/or high water bills, then reusing greywater is important. If you live in an area of higher rainfall like Aberdeen, then reusing greywater may not be necessary.
However, a rainbarrel is useful in any situation. It's convenient to have an outside water source for watering plants and it can be attached to an outside sink which soiled gardener's hands appreciate. In any case, reusing water is cool and it's fun.
if you live in an area of high water bills and/or low rainfall, or you think it would be cool to learn how to build greywater systems, then become a greywater guerrilla. At the very least set up a rainbarrel. If you're resourceful it will cost you nothing. I 'reclaimed' my rainbarrel from a construction site.
TURNING RAINWATER INTO CLEANWATER
The major obstacle in water self-sufficiency is converting rainwater into cleanwater. Convert rainwater to cleanwater and your water supply is free. I know about filtration, reverse osmosis, chlorine, ozone, UV rays and solar stills. I know you can set up a system for just under £10,000 that'll give you over 45 kilolitres of storage, and give you a roof washer, ozone treatment, sediment and carbon filters and UV sterilisation. It'll pay itself off in about 6 years. I'm holding out until I learn the free method.