Secular treaties (formal agreements between nations) often included a section about the past relationship between the two partners. This section reviewed what had happened before the agreement.
In this case, the purpose was to review Israel’s successes and failures since the exodus from Egypt. It reminded the people that God had kept his word, even when they did not obey him.
To prepare the Israelites for life in Canaan, Moses reminded them about life in Egypt, their 40 years of wandering in the wilderness, and their past mistakes. He warned them to obey God's covenant. He assured them that God's grace would follow them as they learned to trust and obey the Lord.