This book header continues Mormon’s practice of providing a synopsis of the book at the beginning. The practice of doing so probably came from the large plates. Nephi provided book headers for his books on the small plates, as did Jacob.
The headers on the large plates had to have a different purpose, however. Where Nephi and Mormon had outlines, and knew what they were going to write, that would not have been possible on the large plates, because the future was still unknown. This header confirms that Mormon wrote it, rather than copied it. The header ends with the statement that it covers events “even down to the coming of Christ.” That could not have been known by the large plate record keeper.
Incidentally, it is of interest that Mormon states that the book of Helaman covers the time “even down to the coming of Christ,” because it doesn’t. We don’t get that information until our book of 3 Nephi. It is possible that the large plate book of Helaman did contain that information, but that Mormon shifted to a new source for 3 Nephi and, therefore, didn’t use the account from Helaman, but rather from the personal record of Nephi.