According to a website, cold showers can stimulate the nervous system, the part responsible for fight or flight response. Cold water shocks the system for a stress response.
I think the temperature of the cold water used in the studies is much colder than our tropical tap water of 25 deg C, instead, it is a freezing 14 deg C. link The cold temperature is supposed to shock the sympathetic branch of your autonomic nervous system. This branch is responsible for the fight or flight and other response.
We spend most of our time in air-conditioned environment, our bodies never get a good workout anymore. The shock of the cold water is supposed to stress our bodies regulatory system.
Personally, I wanted to season myself to the fight or flight response, so I started taking cold showers.