In Vonnegut's Slaughter-House Five, the back and forth and in and out nature of time in the novel reflects what the Tralfamadorians say about time and humans. It demonstrates that they are correct about the lack of human insight.
The aliens say that humans see only in one dimension. They, on the other hand, see all experience at once. The point is that humans do not consider the consequences of their actions. Since we do not see the past, present, and future all at once, we pay no attention to the results of our decisions and actions. In other words, we are figuratively blind. Time experienced all at once is the state of existence in the novel. But humans seem incapable of this.
This plays into the distance between Vonnegut and the narrative, including the protagonist, Billy. Vonnegut uses understatement and detached irony. He observes and relates the story as the aliens seem to observe Billy. The story speaks for itself. The narrator doesn't have to amplify. Vonnegut narrates with detachment. His refrains like "So it goes" demonstrate this. He matter-of-factly uses understatement, which creates verbal irony. Understatement is only possible if distance exists between the narrator and the subject matter. It the narrator is emotionally involved and close to the subjects in this novel, he would have to amplify and elaborate. Understatement is often more effective.
Detached irony allows Vonnegut to let the reader fight mentally for the meaning, rather than for him to have to convince the reader of meaning. Fragmented time and detached irony allow the reader to "put the pieces together."