Could someone provide an example of question about any novel (any book that works for you), a question you still have about the theme, conflict, plot, characters, etc.?

In Frankenstein, by Mary Shelly, why did the author decided to have the character Walton and his letters to his sister frame the novel when he has no real role in the plot itself?

(An answer to this would be that his presence give Victor a reason to be telling the story-as in he is given a meaning to the story and the way that Victor decides to tell it as a cautionary tale)