MarxistFunny story, I was reading a health book, for high school students, from 1908 and it too recommended cold showers to "exercise your capillaries" now while I doubt that's actually a thing that's possible to do
This is one of the main health advantages of cold showers. Your cardiovascular system goes through significant work to literally retract inwards when you're suddenly faced with extreme cold. And unless you live in an extremely cold climate and decide to go out under dressed, this response is very infrequently exercised. Also good for the nervous system I've heard, but I know less about that.
I took cold showers for like 5 months straight in 2017. I felt amped after the showers and it made it really hard to feel cold at all in the hour or so after, but I definitely can not report any of the results like others about increased productivity and especially not it influencing social confidence. Great for waking up the brain and lowering your cold threshold, dunno beyond that