In fact, if you seriously want to catch the biggest fish in the river or lake with a fly, then leave the dries and nymphs in the box and fish only streamers. That's how deadly streamers are. Why not streamers is the question. 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.