Would suggest anyone interested in this topic pick up a copy of Schwiebert’s _Matching the Hatch_. His book published in 1955 started the “matching the hatch” movement and the precise imitation school.
The matching the hatch philosophy was developed as an answer to fishing to selective trout during a hatch event. If you have never experienced selective trout, it is hard to understand how important the concept can be.
It is more important on some streams than others, but I would venture to say all trout waters can produce selective trout at times where the fish key in on naturals, or a stage of the naturals, to the exclusion of all else.
I see stocked rainbows regularly key in a some hatch events here on the White. It does not take them very long to learn what to eat. Fortunately for most anglers, those times are rather limited, but a person who has some skill with matching the hatch can find exceptional fishing during times that prove difficult for the casual angler.
If anyone is in the Tulsa area, I will be speaking on some aspect of this subject at the June 14th meeting of the TulsaFlyFishers.org. Would love to meet anyone from the board that can come.