Deuteronomy means the Second Lawgiving.
Moses is rounding off a preface to the book.
He reminds the Israel what God is like. He is holy, the Israelites saw no form, and must not make anything to represent him. There was a danger of contamination by the false gods of their neighbours. There were periods of repentance, but things went from bad to worse, and went into exile in Babylon.
Paul in Romans 5:10 says that the righteousness of Jesus is so much better than that of the Law, but God still expects undivided devotion. But he is merciful.