Ribbing a fly can be done with many different materials and it can serve several functions. A mylar rib can be added to a streamer body for decoration. Thread can be used to give the look of segmentation as on a natural insect. Wire is often used to rib a fly because it provides segmentation and a little bit of shine. Flashabou does the same yet adds some iridescence as well.
Counter wrapping the rib not only adds segmentation and shine but also helps to protect delicate materials below.
On some flies, like the Slumpbuster, the ribbing does everything and even holds down a second body material, in this case, a rabbit zonker strip.
Ribbing a fly is very simple. To begin, secure the material, here, wire to the hook shank. Then create the body of the fly and finish by making evenly spaced, open spiral wraps over top with the ribbing material. Finally, secure the ribbing material with your tying thread and snip or helicopter the excess off close.