Sefer Devarim is called Mishneh Torah-The repetition of the Torah. Mainly because most of the laws contained in it have already been said or hinted at in the first four books. But perhaps we can see a deeper understanding. When a person is Nifter the relatives left behind learn Mishnayos for the sake of that departed soul. The reason for this is that the letters of the words Mishneh are the same as that of the word Neshama. This highest form of Gematria is when the letters of two words are actually the same but just rearranged It would then come out that calling Sefer Devarim - Mishneh Torah, means that this Sefer is the Neshama of the Torah, the soul of the Torah. How do we understand this?
The soul is what gives life, what animates the physical body. The first four books are the body of the Torah. Sefer Devarim is the soul. It is the repetition of the written Torah expounded by man. The Torah unread and unlearned has no life of its own. However what Moshe accomplished in his final days was to set the example for us to follow. This is our mission, to give Torah, life. By taking the written Torah and expounding upon it, by bringing out the depth buried in each and every letter, the Torah comes alive.