Semi-translucent: flies tied with CDC have more life-like quality than flies tied with other, more opaque materials. Highly aerodynamic: when cast, the CDC collapses but, at the end of the cast, it expands acting as a parachute as the fly descends to the water. It therefore makes casting easier and fly presentation more delicate.
Highly hydrodynamic: under water every filament pulsates with life even in the gentlest flow or on the slowest retrieve.
Folds like real fly tissue when a fish takes a CDC fly into its mouth. In this way, fish accept CDC flies more readily, and are much slower to reject them, than ones made of harder materials.