Streamers give the fish a change of pace. Trout and bass will almost always take a streamer. Big fish eat other fish, they eat fat swimming creatures. They have to because to maintain their size, they must become piscavores. That means hunting for prey that provides lots of calories.
Streamers are big and imitate fish and other high-calorie snacks.
Fishing streamers is the spin fishing of fly fishing. The least important aspect of streamer fishing is casting. The most important facet is the presentation, and that is accomplished in much the same manner as in spin fishing with the art of the retrieval. In streamer fishing, you've got a big rod and big weighted flies. To use a boxing analogy, if fishing with dry flies is the art form of sticking and jabbing, streamer fishing is the equivalent of two heavyweights slugging it out in the middle of the ring.