@Reallyreallyborednow , if you Google it, there are quite a few articles about it. Improved circulation is one supposed benefit, but I think the biggest one might be mental health benefits.
I’ve only ever immersed myself in very cold water a couple of times. Once was when I jumped into a pool under a waterfall in Spain. Another time was at the thermal spa in Bath where you are invited to pour a load of freezing cold water on your head straight after you’ve been in the steam room.
Both times I definitely got a sort of ‘high’. Just felt really alive and refreshed. Endorphins, I guess.