What is the psychology between brothers and men who share wives/girlfriends?

What is the psychology behind brothers or men who share their wives with each other?

When I say brothers, I do not mean literal blood brothers, but just as a “brotherhood”.

I’ve read some things between men in the military, firefighters, police, etc sharing/swaping wives.

They say the reason behind it is for protection, that if the husband or male ever passes away from the line of duty- his wife will be taken care of by someone he already knows & trusts.

I asked one of my guys friends about this and he said he understood because between his brothers and him, they share everything. They are like a family.

Before anyone gets their feathers ruffled, this is just a question and by no means am I saying this is wrong or correct. I’d just like to ask the reasoning behind it…. and if it’s normal?

P.S- This is something I heard while listening to Joe Rogan’s podcast but I can’t find the video anymore :-(