Always remember big fish eat little fish
Streamers are a classification of sub-surface flies that are neither nymphs nor wet flies. They generally reflect a smaller fish that game fish eat, or they could just represent a general source of food, like the philosophy behind ‘Attractors.’ At different times streamers have fallen out of popularity with the fly fishing populace, because they didn’t represent a "fly.” One should always keep in mind that fish don’t have an elitist approach to survival, they will eat anything and everything presented to them. And fish eat other fish, and more specifically bigger fish eat smaller fish.
Another nice benefit of fishing streamers, fish are territorial, therefore many strikes are out of aggression. You are unlikely to receive aggression strikes from using a #16 Pheasant Tail, but when fish see an Aggravator or Woolly Bugger invading their ‘turf’, they might feel a primitive compulsion to strike at it. A word of caution here, streamer strikes are bound to be vicious. Whether striking out of aggression or feeding, game fish know they need to strike fast, as the smaller fish also move fast. This is especially important to remember when fishing for Steelhead, Salmon or other species that might not be actively feeding, but are in a very territorial mindset.
Streamers also allow you to be less knowledgeable about the spot you are fishing. When fishing unfamiliar territory, I often start with a streamer. I don’t need to know any specifics about the hatch, I know there are always bait fish present and they are always ‘in season.’ When fishing in the winter I predominantly use streamers, as hatches tend to sporadic, and game fish are often ‘off their feed’. But they will strike at a streamer when placed down in front of their nose.
While fly fishing can become a fairly complicated endeavor, with all the hatches, the sizes and the colors. A real simple thing to keep in mind is fish eat other fish. And the group of patterns called streamers, take advantage of this fact, and are a real treat for both the fish and fly fishing angler.