In most Japanese bathrooms, you will find a control panel like these, equipped with several buttons. The main functionality of the button is to fill the warm water onto your tub. Once the water reaches to the certain level, it will automatically stop.
The reason why you need this button, is because you are most likely not present while you are filling the bath. Japanese bath tubs are relatively deep, and can contain quite a lot of water. So a typical Japanese family will fill up the tub before anyone enters. Once the water is filled, the water is to be used by everyone in the family. Another frequently used button in the control panels is the water reheat button; once the button is pressed, then the system automatically reheats the water that is currently inside the tub up to the defined temperature, e.g. 40C.
The same button is placed in the kitchen, too. This is for filling the water remotely, which is convenient, but a little stupid because the panel is purely for the heating and does not tell you whether the bath lid is open or not. Nowadays, the control panel also comes with a Talk Button, which works like a bad walky-talky.
When I look at this set up, it makes me feel that the manufacturer has a certain stereotype of Japanese homes. In the evening, mom is busy cooking and dad taking a bath with a child in a bathroom. Considering that there are so many single flats, I feel we need a little diversity in these control panels that occupy millions of households in Japan.
