Insect entomology for fly fishers is especially important if you want to tie your own original patterns for specific hatches you'll be fishing. There, it helps a lot to know the different parts of the bug, how they move, when they move, etc. Of course this is all useful in fishing as well. I encourage every angler, no matter what experience level, to observe the natural bugs on the stream. If you're just tossing a fly pattern because that's what a book told you to use, you're missing some of the richest & coolest parts of the sport.
These days there's tons of info about the mayflies & caddis that trout like to eat available at the click of a computer mouse. Still, it's good to have some reference materials on your shelf that you can go to anytime (or take with you on a trip). There are two books about fishing bugs I highly suggest every angler own.
The first is called "Hatches II" By Al Caucci & Bob Nastasi. This is a fantastic book that does more than just tell you what mayflies are hatching & when. It takes you straight through the entomology, patterns, & behavior of every major mayfly hatch in North America. It is far & away one of the best books I own.
What I like about entomology is what I like about all the other aspects of fly fishing: you can go as deep (or as crazy) with it as you like. There's something in this sport for everyone & entomology is one part of it all. If you want to run around the stream with a big net & a box full of small jars, have at it!! There's nothing wrong with following your passions!!
So no matter how much we get into it, we can all learn more about the bugs the fish we're trying to fool eat. Those two books & that website listed above are great resources for doing just that. It won't be wasted time for sure, as the more we learn about those insects, the better anglers we become.