Interestingly, Jesus places his teachings to the Nephites on this occasion on a parallel with Isaiah. The argument was developed as follows:
(verse 2) Isaiah spoke of Israel, and his words will go to the Gentiles.
(verse 3) Isaiah’s words (which were written) will come true.
(verse 4) Therefore (!) the Nephites should write what Jesus says so that it too will “go forth unto the Gentiles.”
It is hard to imagine that it would not occur to the Nephites to write down this experience with their Messiah, so the message must be directed to the future use of those writings rather than just the act of writing them down. The Savior is encouraging them to write for a future purpose, just as Isaiah’s writings serve a future purpose.