When his sons left, Mosiah was left without a political heir. Even though the sons were alive, they were out of communication in a different country. There was no knowing when or if they would ever return.
There is nothing in the text that indicates that Mosiah was growing old, so there is no indication that a change in the ruler was imminent. However, in a monarchy, succession is important and occurs with greater continuity if there is an heir in place. Without a clear heir, the society is opened to the probability of jockeying for political position. Clans with some claim to the throne would begin to assert themselves against other claimants. Even without open rebellion or civil war, civil discord was virtually assured. It was not a condition that Mosiah wanted to accept.
Mosiah therefore sets in motion events that would establish the continuity of government and forestall the political unrest. As he begins to tell that story, however, he is pulled into a tangent for the next nine verses.