A number of years ago, Rav Aba Wagensberg spoke out in a shiur that Sefer Devarim represents Moshe Rabbeinu’s Mussar to B’nai Yisrael as the time of his death drew near.
He explained that near the end of Sefer BaMidbar, we began to see indications, via the story of Zelafchad’s daughters, as well as the desire of Sh’vatim Reuven and Gad to settle east of the Jordan, etc., that the B’nai Yisrael finally desired to accept Moshe Rabbeinu’s teachings and his Mussar which was meant to bring about the perfection of the Jewish people in emulation of the ways of Hashem as they were about to enter Eretz Yisrael.