Most of All the King's Men is set in the 1920s and 1930s in the American South. For almost an entire chapter, though, we are plunged backwards in pre-Civil War times. This antebellum narrative seems at first to be about adultery and suicide, but we soon learn it has much more to do with the institution of slavery in the American South. When we notice how all the black people in the novel's "present" are either servants or cotton pickers, we realize that this "old" tale has extreme relevance in the here and now.
Though Jack doesn't understand how the Cass Mastern story fits with the other stories in the novel, if you remove Cass's story, none of the other stories resonate as deeply.
Jack's extreme response to the Cass Mastern papers is because that kind of information was being actively hidden from the public in those days.