Vonnegut's Slaughterhouse Five is so weird that I didn't get it at first, but like my run in with Catcher in the Rye a couple of years ago, by sticking with it, I got a sense of something superbly defined and yet inexplicable - and that inexplicability is the narrator's place in the story. In Catcher it was Holden himself, who is narrating his story not just as a narrator narrates to the readership, but to his counsellor/psychotherapist (or so I learnt through a couple of readings).
In Slaughter we have a narrator who plays a role in the fiction, at least 1% of it anyway. He's not speaking to any professional though, but rather the readership.
Two different books, two different narrators, two different audiences - and yet, I believe, understanding these narrators, their purpose for speaking out, and their audience is key to understanding the text and appreciating the nature of the story - one I didn't get entirely for Slaughter, at least not at first.
The story itself is, wacky (though I suspect it had a hand in providing Audrey Niffenegger (though she makes no reference to it on her website - pah!) with part of the idea for the Time Traveller's Wife), all over the place with its time travelling, and filled with moments of disconnectednedd - protagonist Billy, of course, is persistently separate from his travels and seems like an observer... even the narrator makes the statement that little or nothing much happens - no conflict certainly (funny for a book on war).
It stays with you some time after, though not really to do with the Tralfamadorians or their 4 dimensional observations. It has to do with the talk of death, so it goes, and sheer multitudes of people who lost their lives, so it goes... So it goes, is a narrative trick utilisted by the narrator to delineate the idea of how sad and unfortunate but entirely unnecessary all these deaths are, so it goes.
Wikipedia says: Vonnegut used the chorus "So it goes" every time a passage deals with death, dying or mortality, as a transitional phrase to another subject, as a reminder, and as comic relief. It is also used to explain the unexplained. There are 106 "so it goes" anecdotes laced throughout the story.
So it goes stays with the reader as they go, reining them in with this familiar line in a way that, I believe, helps keep the reader's interest when the story wains - it breeds familiarity and gives the narrator some humanity.