Mormon decided that he was far enough off track that he wanted a new chapter beginning for the story of the political change. He had begun to tell the story when he noted that he gathered the records. This verse provides the repetitive resumption that indicates that the insertion of tangential information has ended. We are back on track.
What Mormon had intended with the introduction of the records was to show that they were given to Alma, the son of Alma. That same Alma the younger will become the first chief judge. We cannot know if Mormon tells this part of the story first because it came before Alma the younger was seated as the first chief judge, or if Mosiah prepared the records because he knew that Alma would be the first chief judge.
There is no chapter break here. This was the introduction to the ending of the book of Mosiah. We know that ending as Chapter 29.