Why did Moses write the song in the book of Deuteronomy?
Moses wrote the song in Deuteronomy so that Israel would remember. It was given after God declared He knew their imagination and what they would do before He brought them into the land. The song served as a reminder of God's words and warnings to the nation.
Les explains that Moses, therefore, wrote this song. The context shows God saying He knew Israel's imagination and what they would do even before bringing them into the land He swore to give them. Moses gave the song for Israel to remember. After the song, Moses' speech ends, and God tells him to go up Mount Nebo, look at Canaan, and die there, as Aaron died on Mount Hor. Les also mentions a statistic that in the last 800 years one out of every two Jews faced a violent death, illustrating how God had foretold long before they crossed Jordan what would happen to them.
And it shall come to pass, when many evils and troubles are befallen them, that this song shall testify against them as a witness; for it shall not be forgotten out of the mouths of their seed: for I know their imagination which they go about, even now, before I have brought them into the land which I sware.