A lot of us come from a trout fishing background and also are relied on to fish behaving in a rather foreseeable way. If there is a caddis hatch going on and you can literally view trout proactively feeding for caddis there is a good possibility that if you use a pattern imitating a caddis you're visiting catch a trout. There is a direct as well as apparent connection between just what fish are feeding upon and exactly what will offer you the ideal opportunity for success.
No one actually knows why steelhead take flies. Like salmon, once steelhead go into fresh water they quit actively feeding and simply rely on fat storage to make it up the stream, generate, after that return to the sea where they will gorge themselves and balance the long quickly in the fresh water. So it's type of baffling that we fish using replicas of insects and lure fish that they don't also consume in the ocean yet are prolific to the rivers.