Our chapters 11 through 15 of 2 Nephi were all a single chapter in the 1830 edition. This chapter was set off because it precedes the quotations from Isaiah which Orson Pratt wanted to keep in the same chapter and verse format as in the Bible. Nephi did not set off this introduction from the quotations themselves.
Verse one has Nephi returning to his own account after copying Jacob’s sermon. That sermon fittingly ended with Jacob’s testificatory amen, and therefore a chapter ended. What Nephi indicates in verse 1 might be considered a conclusion to what Jacob had said, and therefore a modern writer might append it to the end of Jacob’s sermon. Nephi does not do this, both because it has the testificatory amen, and because he was shifting from copying to creating his own text.
The only reason that Nephi gives us for the long inclusion of Isaiah is that Nephi’s “soul delighteth in the words” of Isaiah. However, he also indicates that he will liken those words to his own people.
The next thing he notes is that Isaiah had seen Jehovah. Then he indicates the he, Nephi, has seen Jehovah, and then adds that Jacob has also seen him. This establishes through the law of witnesses the truth of Jehovah, including the truth of his law.
What is the point of this? Nephi says that he will liken Isaiah to his people. He has just included a sermon where Jacob had done just that. Therefore, we have these chapters of Isaiah because Nephi intends to use them for the benefit of his people. They are the foundation upon which his later text, beginning in Chapter 25, will be built.