The Words of Mormon were apparently written near the end of Mormon’s life for the purpose of connecting two major records. It was made known to Mormon ‘by the workings of the Spirit of the Lord’ that the small plates of Nephi (which ended when Benjamin was a relatively young man) might be used to replace his abridgment of the book of Lehi [the first book on the large plates of Nephi] (which ended when Benjamin was an old man about ready to die).
So that a gap would not occur in the history of the Nephites, Mormon included the major events of the lifetime of King Benjamin in The Words of Mormon, thus connecting the account on the small plates of Nephi with Mormon’s abridgment of the book of Mosiah.
(Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon, pp. 171)