If you are going fly fishing, you need to have your critical gear with you while wading and casting. One of the critical pieces of equipment is your vest.
Construction workers have tool belts to hold everything they need while working. Fly fishing anglers have the same thing in the vest. They hold your tools of the fly fishing trade. Without them, you will be endlessly hunting for flies, repellant, grapplers and so on.
This necessary fly fishing clothing comes in a variety of styles. Old school vests are the best in my opinion. They fit and look like regular clothing vests, but are made with mesh pockets and water proofing material. I find they are perfect for holding the necessary fly fishing gear. They are also good because the distribute the weight of your junk