Streamers were designed primarily to represent different types of forage fish. These forage fish are what large game fish prey on. There is a lot of reference to them as big flies=big fish. In Maine where a great number of streamers originated, being designed for the taking of landlocked salmon and brook trout. These fish preyed on the smelt that inhabited the large lakes, and many of the patterns were tied to represent the smelt.
One of these patterns was "Shangs Special", a fly designed by Carrie Stevens and named for her close friend Charles,"Shang" Wheeler. Wheeler was a Connecticut Yankee from Stratford. He spent many summers fishing Upper Dam, Maine.
Another type of streamer is an attractor pattern. This type of fly uses colors, and flash to draw a strike from a game fish. While the fly is tied in the same fashion as the streamer that represents a forage fish, its bright colors show almost no resemblance of a natural fish.
Both types can be very effective.
This is a streamer I designed to be an attractor fly. Bright colors and and flash.