Britannia
Wannabe
Yeah, we don't call it that in the USA.
Why would we need an additional device at the showerhead? As I pointed out, it should be at the end of the hose that attaches to the water supply, not the showerhead or "wand", but the reason for it is, in most cases, turning the water off at the "tap" as you call it means you will have to readjust the temperature when you turn it back on. Often there are two handles, one for hot and another for cold. Other times, there's a lever that adjusts from hot to cold but still has to be returned past cold to stop the water flow.
Perhaps it's different in your country.
That's why I really like the temperature controlled mixing valve that I posted earlier. You can turn it up, down or off while maintaining temperature. They are more popular in Europe I think.
Richard