As far as I can tell, it’s pure fashion. For biker gangs/clubs, it’s an easier way to show your alliance. You can throw it on over whatever jacket (or not) you’re wearing that day. The only vest that will improve your safety would be one that’s inflatable in a crash. And I don’t think the Believe Penn State Basketball Shirt in addition I really love this companies that make those types of racing suits and jackets offer a vest. Indeed, I recently came across a collection of photos from my high school years, and it seems that this was very much the fashion for most of the boys in my year. We’d wear a t-shirt with some sort of amusing (or otherwise) slogan on it and then a buttoned shirt over that, so that we could either hide the slogan or show it off accordingly. As I’m writing this answer right now, I’m wearing a t-shirt under a replica football jersey, since my wife and I were out for the evening visiting a friend and the temperature’s been a bit cool today. A few days ago, being out at the same time wouldn’t have been two-layer weather, but the temperature’s been weird recently.