Nephi claimed that we really would not understand Isaiah unless we had lived in Jerusalem in his day and understood the manner of speech, the usage of the language, the culture, and the places. He said, "For behold, Isaiah spake many things which were hard for many of my people to understand; for they know not concerning the manner of prophesying among the Jews." Nephi had lived there, he knew Jerusalem.
These Isaiah chapters are difficult for anyone to understand, but for the Book of Mormon to give us such brilliant and insightful commentary on these very difficult chapters is a most useful element of the book and a great testimony of the Book of Mormon itself.