Now therefore write ye this song for you, and teach it the children of Israel: put it in their mouths, that this song may be a witness for me against the children of Israel.Deuteronomy 31:19
"The book of Psalms was essentially a songbook for Old and New Testament Jews, while other songs are scattered throughout Scripture written by a variety of prophets and leaders.
Our text tells us that the Lord commanded Moses and Joshua to write aspects of the Law and details of God's dealings with the nation, as well as His promise of judgment, should they disobey--in a song.
This song would serve several functions.
First, it would be a memory device. "It shall not be forgotten out of the mouths of their seed" (v. 21).
The second function of Moses' song would be to convict those in disobedience (32:7, 47, etc.).
Thus, the third and evidently primary function of this song was to "testify against them as a witness" (31:21). Much of this song carefully explains their coming apostasy and inevitable judgment. No doubt many remembered this song and its message with tears as they marched into captivity, unable to charge God with unfaithfulness." JDM