The fifth and final book of the Torah, the Book of Devarim (Deuteronomy), is unique. It is written in a style different from the first four books of the Torah. For example, new sections of Mitsvos in the rest of the Torah begin with the words “And G-d spoke to Moshe saying”. The book of Devarim, however, has Moshe talking directly to the Jewish people.
What is the significance of the contrast of the Book of Devarim with the rest of the Torah?
According to some opinions the Book of Devarim is a bridge from the written law to the oral law. While the Jews were in the desert they had an unprecedented relationship with G-d. He literally took care of the Jewish people (food fell from the sky, clouds protected them from the elements, etc…). Their leader at this time, Moshe also had an unprecedented relationship with G-d. He was the only prophet to speak to G-d face to face (all other prophets received their prophecy in a dreamlike trance).
Once the Jews entered Israel, they would not see the hand of G-d so explicitly, neither in their relationship to G-d, nor in the leader who had such open communication with Him. In order to make their relationship work, it would take more effort on their part: now they would have to seek out G-d and his messages themselves.
The book of Devarim is the word of G-d, but repeated by Moshe, whereas the rest of the Torah G-d talks to the Jewish people directly through the throat of Moshe. This would help prepare the people for the reality in the Land of Israel (and thenceforth), where they would hear prophecy, but the prophets did not have the same clear vision as Moshe.
Similarly, the Talmud would be a bridge between those who had seen the Temple, the Sanhedrin, and other aspects of Jewish life, and those who had not.
Nachmanodies, the 13th century Spanish Sage, explains there are 3 things that we learn from the beginning of Devarim where much of our past history is recounted:
1. To show us all the loving kindness that G-d performed on our behalf. This is one of the most important principles of the Torah: gratitude. Without gratitude both to G-d for all that He does, and to our fellow man, we are nothing.
2. To learn from our past mistakes. This shows the flawed perfection of our humanity, the ability to correct our errors.
3. To let us know that although there will be rough patches, when it seems that our sins are overwhelming, G-d will never forsake us.