Home Design Guides: Sustainability, Energy and water management, Energy efficiency, Plumbing
In most plumbing setups, it takes a while for hot water to reach the hot tap when it's turned on. Cold water diverters recycle the cold water which would otherwise pour down the drain in this time.
Just because water from the tap has been used, it doesn't mean it can't be used again. See how grey water systems can help reduce your water waste.