Redaction: These three verses continue Nephi’s prophecy, which Mormon is applying to the Amlicites’ mark. In repeating these lines, Mormon interestingly begins each with “and again.” He is emphasizing that this is a quotation, not his own words. Mormon is simultaneously highlighting the specific declaration and also stressing that Yahweh is the speaker. Structurally, he ends the quotation by spelling out: “and these were the promises of the Lord unto Nephi and to his seed.”
I assume that the written script on the plates followed general practices of ancient texts and lacked quotation marks, paragraphs, and indentations. In such a text, the verbal signals are more important than the visual ones. (See 1 Nephi, Part 1: Context, Chapter 5, “The Organization of the Plates”). It is also likely that the primary audience for the written word would not read it themselves but would have heard someone read it aloud. In such cases, verbal signals like these make it easier to follow the difference between quotation and narrative.