Why is an animal that eats wood like we do the same with a BigMac the best friend of a fly fisherman? Because it cuts down trees. This trees fall in the water and build perfect habitats for bait fish and their predators like the pike. So the beaver creates structure in a water.
But why does it makes so special for fly fisherman. That’s easy. Because the normal fisherman can’t handle all these structures in the water. Spinning fisherman are scared to fish there, afraid to loose all their expensive spinners, wobblers and jerks. Of course this is a brutal generalization, i also know exceptions but that’s in many cases the fact.
The fly fisherman has the opportunity that he can’t fish a lot of spots without a utility that helps him to get in position. The position in the water, the belly boat is such a utility. So from a disadvantage to an advantage. I regularly pike spinning baits from trees that has been broken of from their owners just because they can’t come from the water side. This is way many spots do not have a high fishing pressure and offer for clever fisherman great opportunities for unspooked fish
Have a look in your waters and think about fishing spots where everybody is scared to fish.