The late Sylvester Nemes is credited for the popularity of the soft hackle fly in American fly fishing today and wrote the handbook on these flies, The Soft-Hackled Fly, written in 1975.
Swing it, strip it, nymph it, put some floatant on it and fish it on the surface, there is no wrong way to fish a soft hackle.
Simply put, every serious angler should have one in their box.