It’s similar to movies that start with explanatory on-screen text or voiceover. I mean, I love the Star Wars movies, and I know Lucas was deliberately referencing old serials in the first one, but I feel like Ep. IV would have been a better film without the opening crawl.
A book that starts with a listing of the cast of characters or (worse) a glossary of some kind makes me immediately suspicious. It seems like an open admission that the author lacks confidence in their own ability to tell the story, in the audience’s ability to follow the story, or both.