Even more than that, if you've put a significant amount of your time, energy, money and even identity into a creation, you want for the photos people take (and potentially share) of it to be ones where you have some degree of control over how it looks.

Maybe you cut a few corners on the side of a piece and only want to show it from the front, or maybe the makeup only really "works" if you're making the right expression, that kind of thing. It's a professional courtesy to allow cosplayers at a convention to choose how they pose, rather than unilaterally collect them without permission like a Pokemon.