The two answers sound right to me.
There used to be another reason -
some 2 or 3 decades ago, in Britain anyway, there were shower units
that heated the cold intake as it passed through/across some kind of
electric element situated behind the main panel: I had one myself
installed to replace the bathtub. You pulled a cord with a red ball on it and
the one with a black ball to switch off. There must have been a
control to adjust hot and cold flow, but I don't remember it.
Anyway, if someone ran a cold tap in any other room, the resulting
drop in pressure would cause the heating device to automatically
switch off as a safety feature, so you were standing suddenly under a
spray of freezing cold water ..... great fun.

--
Ian
Ft Worth, TX