From a historical perspective, this appears to be a time of not only peace, but of long-distance travel, which was typically for trade. Verses 8 and 9 confirm the trade aspects of this travel. The ability to increase wealth above and beyond what is basically available to most members of a community come through trade with others. In this case, there is a trade established that connects the south and the north, or the Lamanites and the people of the north.
The value of that long-distance trade route with the north appears at the same time as the beginnings of the movement of populations into Teotihuacan in the valley of Mexico. That trade connection will become a very important part of the end of story of the Nephites, and may be the reason that Mormon adds it here. He is preparing his readers for the rest of the Nephite story. Even though we have the remainder of the book of Helaman to go, as well as 3 Nephi, those two books do not cover a significant amount of time. Most of 3 Nephi covers 3 days. Therefore, Mormon is preparing his readers for the aftermath that will lead up to the final destruction.
He is also setting up the events prior to the coming of the Savior as a precursor parallel to the later events. Before the Savior comes, Mormon will have the Gadiantons be the cause of the destruction of the Nephite government, and, therefore, the Nephites as a people. That will also be the story in his own eponymous book.