One of two ancient poems:
The Blessing of Moses in Deuteronomy 33
The Song of Moses in Deuteronomy 32
The Song of the Sea (Exodus 15) is from an earlier time in Moses’s life, while these two poems are like his "final message" before death.
Moses had already written books of the Law as a witness against Israel if they were to turn away from God. But the law itself required at least two witnesses to establish any charge (Deuteronomy 17:6). Moses was then commanded to write down the song as a further witness against Israel (Deuteronomy 31:19).
The song serves as a witness to God's greatness and goodness, especially his kindness to Israel (Deuteronomy 32:10–14). This kindness makes Israel's sinful response even worse, which brings God's anger and punishment. God will use disasters in nature, wild animals, and wars to carry out his purposes. Yet this is not the end. God, in his grace, will turn against Israel's enemies instead and rescue his own people (Deuteronomy 32:36).
This song carries the same message as every great prophet of the Old Testament. Psalm 78 expresses this message through examples from Israel's history. The song describes the very nature of God, so it makes sense that in heaven people sing “the song of God’s servant Moses and of the Lamb” (Revelation 15:3).