The Churn is a short novella which tells the story of one of the major characters from The Expanse series: Amos Burton. I like Amos a lot, but I’m not totally sure I care about having read this; it wasn’t bad in any way, or not enjoyable, but it wasn’t necessary. We know who Amos is, and we get glimpses of where he came from… and I’m not sure I needed to read this novella to fill it in. I almost prefer piecing it together, because in some ways who Amos was isn’t as important as who Amos is… and in other ways, he really hasn’t changed very much in the series: it doesn’t add much development-wise, because the differences are simply in the different contexts.
So, if you’re a hardcore lover of The Expanse and you want to read every scrap of background, then yeah, The Churn is probably of interest. If not, then I don’t think I especially recommend it — the main series are all you need, I suspect.