Billy Pilgrim claims to be captured by aliens and to experience time travel. Are we meant to take this notion literally?
Much of the reason behind Vonnegut writing Slaughterhouse-Five has to do with the feelings he had witnessing the aftermath of the fire-bombing of Dresden. In order to make sense of the event and present it in the way he wishes to, the time travel with the Tralfamadorians becomes an important part of the novel.
By juxtaposing Tralfamadorian society with that of humankind, Vonnegut brings to the forefront the question of whether there is such a thing as free will. In doing so, he further questions the necessity and humanity and reason behind the choice of the Allied powers to firebomb Dresden, something that was certainly not necessary or likely to have a great effect on the outcome of the war.
In doing so, Vonnegut deepens his disdain and disgust for the decision to carry out that attack making it very clear that it was a choice, not something that "had to be done" and therefore purely evil and inhumane, something that most accounts of the war choose to gloss over en route to proclaiming it a good and "just" war.