If you look at the chapter heading for chapter 20, you'll see that it instructs you to compare Isaiah 48. After reading both of those chapters I was really struck by the fact that they are so similar. A lot of the wording is almost exactly the same - and I imagine some of the differences are because of translation from the original language.
So here are these two chapters - revelations given to two different prophets, in two different parts of the world, hundreds of years apart (Isaiah is thought to have been written somewhere in the 800s B.C. and this part of 1 Nephi was around 570 - 590 B.C.). And yet they are almost identical.
To me, this illustrates the unchanging nature of God. God is the same yesterday, today, and forever. All other things may change and pass away, but God will remain.