There is a view that the Book of D’varim is merely a recapitulation of the material in the previous four Books, and therefore Moses is summing up all that he did and said in the years of his leadership.
This opinion cannot be correct, because D’varim actually contains a fair amount of new material. Most of it deals with personal status, especially marriage and divorce.
According to tradition, this material was already known to Moses from his sojourn on Sinai when God gave him the teaching that was not all slated for promulgation until a later stage.
We wonder why there was a delay in providing the people with their marital code; the explanation might be the human passions which drove relationships with the opposite sex.
At that point little notice would have been taken of preachments that tried to restrain people’s emotions and drives, and it needed time to acknowledge that rushing into unsuitable unions needed to be restrained and there needed to be laws and procedures to govern the beginning of marriages.
Likewise, the possibly hurtful abandonment of unsuitable partners needed to be modified and a code of divorce law and procedure accepted.