Most the time, trout are feeding on nymphs under the surface. And there's a lot of water to cover, 'under the surface'. On any given day you might bounce big stonefly nymphs along the bottom, then drift a big pheasant tail in the middle of the column, and end up swinging a hares ear dropper as the hatch is starting up. Nymphs from YD Flies get you to the fish that bite under the surface.
Guaranteed.