@DLC, I think it’s really just a way for authors to help readers keep track of all the characters. The series I mentioned above,
Warriors, by Erin Hunter, has like 30+ books in it across different arcs, including special editions, field guides, manga, and probably more (that was a good 13+ years ago when I was big into the series, so I’m a little rusty with remembering everything from it at this point). There are four clans in the modern setting, and each clan has probably 20 or so members. Even though the narrative mostly just follows one clan (and thus the character list is completely filled out for that one clan), remembering each character (cats) and what their role in the clan is can be a tall task for middle graders. Those lists they can go back and refer to are going to help readers keep track of the characters easier.
And I’m not in any way suggesting, by my example for middle-grade books, that adult books can’t or shouldn’t use this same tactic. Adults start books, get busy with life, and don’t get to continue reading it for a month and forget things about it. Lists like this would help those people get back into the book if there are a lot of characters to keep track of.
@JT Woody, I read Guardians of Ga’hoole, too! I completely forgot about the pictures, but I love that decision they made. I feel like the intention in that specific instance was probably to show middle graders that there are many species of owls, in addition to giving them characters to imagine when reading. Although my books don’t have a lot of characters, the idea of doing something like this strikes me as interesting because I like to draw my characters.