Too be honest I am not sure, but here is my best guess. When I was younger I never really did. When I joined the Dan Bongino Wearing Made In Usa T-Shirt also I will do this Navy it was a requirement. I suspect that for most guys it is just one of those carry over type things that you do after you get out. American here..I know I wear them particularly to contain sweat! I live in Florida, it is hot, and I think it is embarrassing when people can see sweat showing through your shirt. This being armpit circles, or anything from sweat drops to sweat marks on your back. Also, as for armpit sweat marks, those tend to stain your nice shirt, and I’d rather stain my undershirt vs my nice buttoned shirt! (Also, I am skinny.. and skinny people sweat too!)
Buy it here: https://xfrogclothing.com/product/dan-bongino-wearing-made-in-usa-t-shirt/
Home page: https://myfrogtees.com/