I think that question has an element of the old question "Which came first, the egg or the chicken?" I started tying poppers as an 11 or 12 year old since I got a Thompson Kit for Christmas with my first fly rod. My crude poppers were much better than any I could buy simply because they were big enough to entice big "glades' LM Bass to eat and way too big for the then all-present bream to bother with. They also had big enough hooks to result in a high percentage of solid hook ups.
Then as I got into trout fishing with drys up noth, I began tying dry trout flies based on the shape of the naturals as seen from the bottom looking up - and had good luck with those. So I had confidence in them. I also - by then- was using good materials so the drys would float.
After getting out of the service, I began fishing the keys with flies and tying spun deer hair crab patterns with varying sink rates (again, very primitive looking flies compared to bought ones). But they looked very close to a fleeing crab when stripped, and one with both pinchers held up when sitting on a sand or hard bottom. The down side was that one "pincher" was a single hackle tied to the bend just behind hook barb facing forward and was thus delicate and had to be cast gently or it would break off.
So was the tying of my ugly, primitive flies (still) making me a better fisherman, or was observing the behavior of thousands of crabs, pilchards, glass minnows, hard heads, ballyhoo in their own environment primarily responsible? And then tying something that I could fish to immitate their behavior as closely as possible.
Frankly, I wouldn't tie any flies if I could buy ones that I could fish the same way as I can fish those I tie myself.
Turbine, just saw your post after posting mine. Pretty much the same philosophy - except I'd rather not have to tie my own. And if I didn't live alone, I wouldn't be allowed to, either . Most women insist that dining room tables are specifically used for meals.