Cock Rings. Is it a Dom or sub thing?

Hi all. I saw a couple of men walking down the beach in this video; https://xhamster.com/videos/nudes-walking-on-beach-bvr-3873571

I've seen others like it too, but at 3.53 to the end a man is wearing what looks like a metal ring round his cock and balls.

My question is why? Is it a dom or sub thing, something else or is it just for decoration?

There is one at 2.20 too.

I had been told they maintained erections but as you can see, thsts not the case.

58 seconds into this one too https://xhamster.com/videos/cap-d-agde-walking-xednorton-4474696