Sometimes, I tie flies without a special purpose. I just think “I want to tie a fly, maybe an emerger would be cool.” I open some drawers and take a look at my hooks, thinking how the fly (or in this case the ermerger) could look like. Then, I’m thinking about the tail, the ribbing, the abdomen, …., step by step until the fly is finished. I believe when you go this way, you often try something different and that’s the way you improve your skills and knowledge. There are errors, of course, but even errrors improve your knowledge.
A short example: You are sitting in front of your vise and you’ve tied a beautiful mayfly with awesome natural foil wings. Looking really good. You go to the river with the beautiful winged mayfly on the end of your leader. You do the first cast and….. the wings made a propeller out of your fly. Impossible to cast, impossible to fish. The leader is totally twisted after just one cast. Error. You’ve learned something, next time, you will tie your fly different and so you improve your knowledge.