Why would Lehi emphasize the role of gentile branches? Of course, one answer would be simply that he saw that in his vision, and therefore recorded it. However, that is not a satisfying answer, because it simply shifts the question to why God would give him that information.
The key is that Lehi is making this personal for his family. Note in verse 13 that he says “that we should be scattered upon all the face of the earth.” He is certainly part of Israel, but at this point in time, Lehi’s interest is in his own family. Remember that his dream was about his immediate family, not all of Israel—not even about Ishmael’s family.
The reference to the gentiles is probably foreshadowing of the importance of the New World gentiles to the children of Lehi. The concept of the label gentile is that there is a difference, an opposition. Lehi accepts the difference, but makes certain that the gentiles come to the knowledge of the Messiah. Through that process, they would no longer be gentiles, but become children of Israel, children of the Messiah.
Nephi once again signals the end of a substory by noting that “all these things … were done as my father dwelt in a tent.” He is ready to begin a new story, but not a new chapter.