Spook Less. Hook More. Land More.
Silk fly lines are much thinner than PVC floating fly lines. It's no coincidence that older cane & bamboo fly rods had smaller rings. They were also softer in action due to the materials. Rods didn't need all the power they have now because the lines were thinner and cut the air much more easily.
Reducing the diameter of the projectile has a profound domino effect on all the other components of the fly fishing system. If the rod you are using is now softer, you have more shock absorbency in the fly rod. That means you can use lighter tippets, which generally means you spook fewer fish and hook more fish. The rod bends more and protects the tippet so you land more fish.