Nephi has seen some of the events of the final war between God and the Devil. However, he also sees that John the Revelator will write more fully of those events. Therefore, Nephi need not write them. Nephi has already seen that the book of his seed will be joined with the book that would contain John’s writings, so he knew that he needed to write with more precision.
Seeing John, however, lets Nephi know that this is truly a witness of the end, and that he and John will each stand as a witness. The law of witnesses, that truth be established through at least two witnesses, will be upheld.