Muskrat has a very fine fur! You can make a unique and superfine dubbing from this fur piece and you can tie a heavy leaded nymphs extremely thin, which is great because fish prefer much more subtle and thin types of flies.
Another indisputable advantage is the origin of this material - Muskrat is an animal living in the water. It is generally known that the fly tying material and fur from aquatic animals are more natural in water - the refraction of light, the binding of air bubbles, etc., against to terrestrial animals.