I do it for the same reason I take pictures of what I paint.
1.) So I can see something that I may have missed and need to work on. I am always finding that in minis I painted, and I find things I miss and could improve on while dressing as well.
2.) So I can track my progression and see where I need to improve. My cat eye and Smokey eye game is getting stronger, but they still need work.
3.) *Bonus* I just like to, even if they are just for me and my wife to share together.