It usually refers to cooking, but it has definite importance with fly recipes too. If I’m tying one or two of a recipe for my own use it really doesn’t matter. When tying sets of 14 for casting for recovery it is vital. For the Los Alamos ants I just completed I had rubber tails in one bowl, legs in another and two colors of precut foam. I had separated out enough peacock herl and 14 hooks. I admit that some materials don’t lend themselves well to precutting (like deer hair for the x-caddis that are up next) but having as much as possible prepared in advance makes for much faster and more consistent tying.
i do that when I’m tying for a fly swap of multiple flies
otherwise, I’m usually tying 1 of and just use materials accordingly