Redaction: Why does Mormon repeat the story already told in Mosiah 8:6–11? Nothing about it returns us to Ammon’s essential story—which is his meeting with and eventual liberation of the Limhites. In the previous story, it fit well into the context but seems out of place here.
I see the most plausible answer as lying in a source shift. Mosiah 8 tells the story from Ammon’s point of view. That story made sense because Ammon knew of King Mosiah’s abilities with the Urim and Thummim. However, in Mosiah 21 Mormon is writing the story from Limhi’s account, which also records the incident of the men who searched for Zarahemla. Limhi includes this detail because of the great contrast between their sorrowful conclusion that Zarahemla had been destroyed (v. 26) and their joy at learning from Ammon that Zarahemla survived.
A modern reader does not need this repetition; but in the context of Mormon’s two sources, both tellings are appropriate. We are left now only with the question of why Mormon decided to include it. I deduce that, although he felt free to provide synopses, he also remains relatively faithful to his source documents and, hence, included the event both times.