Deuteronomy is like AP Israelite History for the ancients. Why does Moses retell all this stuff that we already heard in Exodus, Leviticus, and Numbers? Well, as your history teacher has told you a million times, we can better understand our current situation by looking at what's come before us.
Example? Your ancestors were afraid of giants so they missed out on the Promised Land. Lesson: don't be afraid of giants.
The writers of Deuteronomy are conjuring up these shared cultural memories to give Israelite culture a new lease on life and to remind them that their community shares a cultural narrative and set of laws. But this memory stuff applies to the Big Guy, too. When the Lord wants to wipe out the Israelites and start over, Moses reminds God of the promises made to Abraham, Isaac, and Jacob in Genesis. And so they live to see another day.