Silk fly lines tend to be 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, because you don't need a stiff action fly rod, you have more shock absorbency in the fly rod. That means you can use lighter tippets, which generally means you hook more fish. It's not uncommon to drop tippet strength 50% when using a micro thin fly line.
The trend towards fast action fly rods was a reaction to fly lines getting thicker. Lines got thicker because the market demanded higher floating fly lines and brighter coloured pigments (which add diameter.) As rods got stiffer, fly lines then got heavier, thus the 'line one rating heavier than it's AFTM' situation we have now. We are faced with a hamster wheel situation where rods respond to thicker lines and lines respond to stiffer rods.
Sunray have broken that cycle. By reducing the diameter of the fly line, you can use a rod rating lighter than you would normally need. Or, you can use a softer action carbon fly rod, or a glass rod, or a cane / bamboo fly rod. Also, you can use a smaller, more balanced reel because a thinner line takes up less space, jams against the cage less and reduces fatigue by reducing weight. You can now fish the lightest, most sensitive fly fishing outfits in the world, simply by reducing the fly line's diameter.