Because the Gentiles are now in the land of promise, it is important that they prosper in the land. This is the symbolic effect of the promise of the land. If you are righteous, you prosper. Nephi has previously shown that the Gentiles who came were both protected and victorious. Now he establishes the nature of the righteousness.
Nephi is establishing a parallel between the Gentiles who inherit the land, and the promise, and the Nephites who originally had been given the land of promise. For both, the promise was conditional. For both, it included protection from other nations who battled against them. For the Nephites, it was their brethren the Lamanites. For the Gentiles, it was their “mother Gentiles.” In both cases, the enemies are designated as relatives who turn against them.
Now, the vision introduces a Gentile book. This will parallel the Nephite book.
For Nephi, it was important to show that the book was related to the brass plates. Nephi clearly understood that the children of Israel had scripture. He knew that the plates of brass contained many of them. What he did not know was whether the tradition would continue. He learns that it will. Thus, the word of God begins the same for both the Old and New World peoples, but then the records continue separately in the two hemispheres. Both continue to be the word of God, and will eventually be brought back together.