I believe (of course since I can't ask Kurt himself) that one of the main reasons he digresses so quickly and easily, particularly in creating a very non-linear narrative, is because there is no real good way to approach the really horrifying aspects of the story. One cannot simply tell the story of being captured, locked in railroad cars and shipped across Germany, then witnessing the terrifying results of the fire-bombing of Dresden in a normal, from A to B type of narrative.
I think Vonnegut created these digressions to help create and answer to that dynamic. The reason for the mantra of "and so it goes," is similar. Having been a witness to the terrible power of an incendiary bombing raid and the barbarism inherent in it, there are no more easy answers or answers at all for why someone dies and someone else doesn't. So the simple response is "so it goes."