It's usually done when there are a bunch of objects in a photograph and it's hard to identify them or refer to a specific one without confusion.
For instance, I've seen this done with jigsaw puzzles that have a certain theme, and on the back, there's an outline illustration that has numbers on it that correspond to a numbered list of elements.
I've seen something similar done with mountain ranges, where it's hard for a person unfamiliar with the individual peaks to pick them out.
I feel like it's kind of a "retro" approach, but because I don't know the right words to describe it, I'm finding it near impossible to locate other examples.
EDIT: I just found a better example from this blog post:
They call it a key, which feels right though mostly I keep finding images of literal keys. ;)