Textual: Mormon begins a new chapter with this verse. He has been ending chapters with a statement of the years. This time he also opens his chapter with a reference to the years. It is probably no coincidence that we see more of the year markers in this section of the Book of Mormon than at any other time. When Mormon is writing this section of his abridgement, he is following a chronology, and the passage of time is part of what is important to this story.
Of course Mormon does not always begin a chapter with the changing of a year. There should also be an ending and a beginning. In this case, the ending was the conclusion of the chapter that focused on Moroni. This chapter begins with, and focuses on, Helaman and his young warriors. The next reason for the shift in chapters is that this is an inserted letter. This letter covers from our chapter 56 to the end of our chapter 58. This is a typical reason for Mormon’s chapter breaks.
Chronological: the thirtieth year of the reign of the judges would be approximately 64 BC.