The beauty of fly tying is you can create EXACTLY the fly you want in your preferred sizes & colors. In many cases, it will be a fly you CAN'T find for $4 in a fly shop. You also have the advantage of creating a fly based on a natural you capture and bring back to the tying bench, something I've done MANY times.
In my evolution as fly fisherman I first bought what someone else thought would catch fish. When I started fly tying, I copied the flies I bought. Later I modified the colors of those flies to match the local bugs which even though I don't think it was necessary, it was more fun.
Finally, I started making my own creations based on my own ideas or things I learned from others and got to the point where I am today where 90% of the files I fish you will NEVER find in a fly shop.
That doesn't mean those other flies won't work, but why buy & try when I know I have patterns I can create at home that work for me just as well or better.
BTW - I have big wide hands and fat fingers and I regularly tie flies as small as the TMC 32's. I can also whip finish by hand...
Errr, whip finish
the head of a fly by hand with no problem but I'd rather use a whip finisher...
The only disadvantage to having big hands/fat fingers when fly tying is I jab myself with hook points more than the daintily fingered folks.
If there is an advantage, my wife hasn't yet commented on any correlations...