Answer:
Using a different dialect for the narrator than the characters can serve several literary purposes. One reason is to create a contrast between the narrator and the characters, which can help to highlight the differences in their social or educational backgrounds. In the case of the book you are reading, the narrator's fancier dialect compared to the other characters' southern dialect could suggest that the narrator is from a more privileged or educated background than the characters, and may view the world and events of the story differently because of this.
Another reason for using a different dialect for the narrator is to establish a distinct narrative voice. By using a different dialect or register for the narrator, the author can give the narrative a unique tone or style that sets it apart from the characters' dialogue. This can be especially effective in first-person narratives, where the narrator's voice is crucial in shaping the reader's understanding of the story.
Overall, using a different dialect for the narrator than the rest of the characters can serve as a literary device to add depth to the story and create contrast between characters, as well as to establish a unique narrative voice.