Think about the amount of water that people waste waiting for the hot water to finally make its way to the tap. Yes, it is more effective to wash your hands in hot water, but the alternative is to just use antibacterial soap and wash them longer.
I lived in Japan for 4 years and these were in every home. The biggest thing is that you have to use it when you flush no other on or off switch. Japan has the best gadgets for the home.