Next time you see a band of bikers riding down the highway, count them to see how many are actually NOT wearing leather vests. I would guess that as a percentage, the number would be pretty small. Most bikers wear leather vests or jackets, as well as other pieces as appropriate. But why? What's it all about?
Most leather vest might be merely a vehicle for which to display military patches or club colors, but that's not always been the case. Bikers originally started wearing leather because it offered the most protection in case of a spill. Yet they weren't even the original users of the material. Native Americans wore leather as a matter of practice because it was durable, warm, and very effective in protecting them from all sorts of things that could cause cuts, bruises, and abrasions. Even old-fashioned cowboys wore leather chaps and vests to protect themselves from the dangers of underbrush.
Today's bikers sometimes wear leather for the same purpose. For example, chaps and leather gloves are pretty standard among bikers whose Harley's are their only form of transportation. You'll also find plenty of leather jackets covered with embroidered patches among motorcycle groups traveling in cooler weather. As for the leather vest itself, it doesn't provide a whole lot of protection against the weather or other nastiness. It is primarily just a means of looking cool and displaying your biker patches.
At the end of the day, leather vests are part of the biker culture just as much as tattoos, assorted pieces of jewelry, bandannas, and military patches. It would just be unnatural to see a large group of bikers pull into the Sturgis or Daytona rallies not wearing leather - the biker universe just couldn't deal with it.