Silk lines, braided, coated with linseed oil were create at beginning of XX century. Halford famous fly fisherman look that tapered line was better to cast.
The sophisticated taper was devellopped by famous french caster Pierre Creusevault and Jean Michel Dubos. They gave its excellent aerodynamic qualities, this allows for ease of casting at all distances.
Today Jean-Pierre THEBAULT member of french casting sport team, fly fisherman, graduate in Chemistry from University of Poitiers (France) summarizes all this studies.
Their finer tips (30 % less than plastic lines) are the ultimate for delicate dry flies presentation; their slimmer profil induces less drag and allows for shorter leader / tippets combination.
Silk lines float higher, the dressing applied to the line repels the water making it float in the surface film (plastic line must displace equivalent volume as its weight, thus settling deeper in the water to float) Rollcasting is beyond belief.