The use of Feathers in Fly tying to give your flies Body, Movement and some bulk
CDC
CDC on Dry flies to help them float and stay on top of the water, CDC on Nymphs and Caddis flies to trap some air bubbles and give the flies a realistic look in the water.
Pheasant tail on Nymph flies for the tail, body and wing section, I have used Pheasant tail on Caddis flies for feelers ( https://www.cmffly.co.za/step-by-step-orange-caddis/ )