The Book of Mormon includes books which start or end with a summary of what is about to come, or a summary of what has just happened. These summaries are known as colophons and are found in ancient documents.
John A. Tvedtnes explains:
Hugh Nibley pointed out several years ago, they appear in several Egyptian documents. For example, the Bremer-Rhind papyrus opens with a colophon that gives the date, the titles of the author, genealogical information about his parents, and a curse upon anyone who might tamper with the document (in other words, an avowal that the record is true). These textual elements functioned in antiquity somewhat like a copyright or seal of approval.– John A. Tvedtnes, “Colophons in the Book of Mormon,” in Reexploring the Book of Mormon, edited by John W. Welch (Provo, Utah: FARMS, 1992), Chapter 3
An example of a colophon is found in 1 Nephi:
An account of Lehi and his wife Sariah, and his four sons,” and ends, “This is according to the account of Nephi; or in other words, I, Nephi, wrote this record.”
Why would Joseph Smith go to the trouble of adding them at all? It certainly would make dictating a book on the fly much more difficult. It would require either accurately stating in advance what is about to happen or accurately describing what has happened. Why would Joseph Smith make the Book of Mormon more difficult for himself by including these unnecessary colophons?