As we discussed in "Genre," Slaughterhouse-Five really draws attention to the fact that it's a book, being written by an author. This is part of what makes it a "postmodern" novel (see "Genre" for more on this). With a more traditional novel, you read it as though it's a direct representation of something that really happens (even though you know it's fiction). In other words, you forget that you're reading a novel. But readers of Slaughterhouse-Five can never fall into this illusion. The narrator introduces the book and pops up throughout its "fictional" sections to remind us that "Billy Pilgrim" is a product of his imagination. At the same time, the blurring between the narrator's life and Billy Pilgrim's experiences challenge our ideas of what is fiction and what is truth.
The narrator appears occasionally in the "Billy Pilgrim" portions of the novel to remind us that, while the events Billy encounters in Germany may seem farfetched or unfamiliar, they are based on the narrator's lived experience.
Vonnegut uses the fictional structure of the Tralfamadorian novel to structure Slaughterhouse-Five. This emphasizes that both the narrator's introduction to the novel and Billy Pilgrim's portion of the story arise from the same place: Vonnegut himself.