Slaughterhouse isn't exactly your traditional war novel. From my personal experience with war stories, they are very linear and connected. Events take place in chronological order and usually the "war story" is the main part of the novel. For one, Slaughterhouse Five has a separate subplot about Billy Pilgrim’s kidnapping by the Tralfamadorians. There is a tremendous amount of jumping around as the novels skipping timeline advances the story of Billy's experience during World War II with flashbacks of important events throughout Billy's life. In my personal opinion, all of these quirks make the experience feel more genuine and real for a reader. When I read Slaughterhouse Five, I felt like I was talking to a war veteran telling me about their war related experiences while also telling me about their life story. If I didn't know the veteran, it would only be natural for the person to tell me both together.
In Slaughterhouse Five, Vonnegut is recalling experiences from a very difficult and disturbing part of his life. Most war veterans who have experienced similar atrocities don't want to recall the events they experienced. Often times people try and forget terrible things so they can move on with their lives. These types of events are difficult and a "touchy subject" for many to recall and usually people tend to come out with bits and pieces slowly rather than entire narratives. The odd chronological structure of the book is explainable and makes it seem genuine. The way it skips back and forth makes it seem like you're talking to someone who was involved in the war as they are trying to recall various events that took place at various times and then frame a story. The various subplots would be a part of the conversation with this person. When people tell stories, they often digress and tell other stories that relate and then pick back up where they left off. It seems unnatural to tell an entire historical account of what happened like most traditional war stories tend to do.
For someone like Vonnegut who was captured by the Germans and placed in a work camp, experiencing the Dresden bombing is an undeniably difficult time. Witnessing thousands of deaths and immense destruction as a prisoner of war would be difficult because of how much time you would have to contemplate life and try to make sense of the terrible things happening around you. I find it reasonable that Vonnegut(Billy) had so many flashbacks and flash-forwards. These flashbacks and subplots would be an important part of the narrative that someone might tell years after the experience. How someone remembers an event isn't always defined by what happened but what they were thinking at the time. These various flashbacks might relate to things Vonnegut was thinking trying to be in a better place while the war was happening.
