"Frankenstein" and "The Invention of Morel"

Excerpts from “Frankenstein” and the first part of “The Invention of Morel” had similar ideas in their stories. Both of them incorporated things into them that would be under normal circumstances considered impossible. In “Frankenstein” the monster appears seemingly out of nowhere by the body of his dead creator. In “The Invention of Morel” either invisibility or time traveling seems to be in play. The excerpts from “Frankenstein” and “The Invention of Morel” are both written as if they are written to someone else. Walton tells the events through letters to his sister. The main character in “The Invention of Morel” is writing to leave a record of his adventures for who ever happens to find them. I wonder if there is a reason why both these authors choose to have their characters writing down their own story rather than just telling their own story. Does it make the story more believable?