Devarim 28:69 records that G-d made a covenant with the Jewish people in the land of Moav, or more specifically on the plains of Moav (Bemidbar 36:13, Devarim 34:8), and this covenant was in addition to the covenant that was established at Horev, (Sinai). Why was there a need for another covenant between G-d and the Jewish people?
The most logical reason why another covenant is needed is because the previous covenant had been nullified, and in fact, this might have occurred by the covenant at Mount Sinai. It is possible that the sin of the golden calf nullified the first covenant made at Sinai, and hence a second covenant was made at Sinai, see Shemot 34:10. However, this second covenant (or the first if the sin of the golden calf did not nullify the covenant) at Mount Sinai was never nullified. Even by the sin of the spies (Bemidbar 13,14), the Torah does not mention that the covenant had been nullified. Immediately after the sin of the spies, G-d commanded the people about laws they had to keep in Israel, Bemidbar 15:1-41, which shows that the covenant had not been abrogated. Furthermore, 28:69 explicitly records that the covenant in Moav was in addition to the covenant at Sinai, so why was there a need for another covenant on the plains of Moav?
I believe the most popular answer, based on the views of R. Akiva (Sotah 37b), is that the new covenant on the plains of Moav was not new but was a repetition of the covenant from Mount Sinai since R. Akiva maintains that Moshe was taught all of the Torah when he was on Mount Sinai. However, R. Yishmael argues with R. Akiva and claims that new laws were taught to Moshe after he came down from Mount Sinai. Yet, even if one accepts R. Akiva’s opinion that Moshe was taught all the laws of the Torah on Mount Sinai, this does not mean that he told the people the laws of Devarim until the 40th year. If Moshe did not teach the people the laws of Devarim, then it is quite amazing that the generation who experienced the Decalogue did not receive the complete Torah.
26:18, 27:9 and 29:12 imply that the covenant on the plains of Moav was to complete the covenant of Sinai, and not just a repetition of the first covenant. These three verses record that the people were going to become G-d's nation on that day. What happened on that particular day? As R. Yehuda (Berachot 63b) asks, “Was it on that day that the Torah was given to the Jewish people?” Ibn Ezra (on 27:9) explains that 27:9 means that on that day the covenant between G-d and the Jewish people was established, and it was the covenant that made the people G-d's nation. Yet, were not the people already G-d's nation since they already had made a covenant with G-d at Mount Sinai forty years earlier? The answer is that the covenant at Sinai was only a partial covenant, and hence the people were not yet fully G-d's nation. Only when both covenants, the covenants of Mount Sinai and of Moav, were established did the people truly become G-d's nation.
29:8,11,13, also imply that some new covenant was established on that day, and not that the covenant on the plains of Moav was just a repetition of the covenant from Mount Sinai. This new covenant was the completion of the covenant that began at Mount Sinai, as stated in 28:69.
If the covenant on the plains of Moav completed the covenant made at Mount Sinai, why did G-d make the first covenant at Mount Sinai a partial covenant, which necessitated an additional covenant? This enters the realm of speculation, but here goes.
Maybe the first covenant at Mount Sinai was only a partial covenant since the goal was to establish the complete covenant in the land of Israel, see our discussion above on 3:23-28, "A private prayer?" However, Moshe was needed to establish the covenant since he was G-d’s messenger to transmit the covenant, and hence once Moshe was unable to enter the land of Israel, 3:27, then the additional covenant could not have been done in Israel. Instead, the covenant on the plains of Moav was established as close as possible to the people's entrance to the land of Israel. In addition, the ceremony on Mount Gerizim and Mount Eval was to take place immediately (27:2,4) when the people entered the land of Israel, and this ceremony was part of the establishment of the covenant of the book of Devarim.
A second possible explanation for why G-d initially only imparted part of the covenant to the people at Mount Sinai is from 29:3, which records that Moshe told the people “But G-d has not given you a mind (lev) to know, or eyes to see or ears to hear until this day,” (Fox, 1995, p. 987, translation). This verse is perplexing. What was it that the people did not know? Why did G-d not let the people have this knowledge? Also, what was happening that day that would give the people the knowledge that they had been lacking?
I believe the standard explanation of the verse is that the people did not understand G-d due to their intransigence or sins (see N. Leibowitz, 1980, pp. 289-293), but the verse refers to G-d as being the cause of their misunderstanding. Thus, this lack of knowing was not because the people who left Egypt had a slave mentality.
What knowledge did G-d withhold from the people and why? A possible answer is that the withholding was that G-d only partially transmitted the covenant until that day. On this particular day they established the covenant on the plains of Moav, and then the people would have full covenantal knowledge of G-d. 29:3 means that not only did the people lack full knowledge of the covenant until that day, but also they lacked the full ability to comprehend G-d since G-d had not given them the complete covenant.
29:4,5 explain that G-d withheld from the people the complete covenant since they had to undergo the process of living in the desert and depending on the mahn for food every day. Thus, 29:5 ends by stating that the experience of not having real food but living on the mahn was to give the people the knowledge of G-d. The people had to know on a daily basis that their life was dependent on G-d, see 8:3. Also, the people had to know that G-d was with them even if they did not see G-d doing miracles, as the people did not see the mahn when it came down in the nighttime. This process was to teach the people what is means to believe in G-d and this was necessary for the people to be able to receive the complete covenant. Thus, the mahn was to be stored forever to show the people this important miracle, Shemot 16:32. Accordingly, the first covenant at Mount Sinai was not a complete covenant since the people had to experience living based on the mahn until they could receive the complete covenant.
The mahn parallels the ten plagues since both were to give the people a knowledge of G-d, 29:5 and Shemot 9:14-16; 10:1,2. The experience of the ten plagues was necessary for the first part of the covenant at Mount Sinai, and the experience of the mahn was necessary for the second part of the covenant on the plains of Moav.
It could be that the two reasons for why the covenant at Mount Sinai was a partial covenant, the desire to have the complete covenant in the land of Israel and the need for the people to experience the mahn, are complimentary. The people began to receive the mahn on the 15th day of the second month (Shemot 15:1), and they left Mount Sinai, a little more than a year later, on the 20th of the second month, Bemidbar 10:11. (It is striking that the Torah records these days, when it is very sparing with dates, and maybe the reason is that we should know that a year passed from when they first started to get the mahn.) Thus, the people had a year to receive the mahn, which should have been enough time for the lesson of the mahn to be instilled in the people. The covenant that was later established on the plains of Moav was not told to the people at that time since very shortly afterwards they were supposed to get to the land of Israel, and then they could complete the covenant in the land of Israel. However, after the sin of the spies, it was clear that the people needed more time to learn the lesson of the mahn. It is not clear if they needed exactly forty years, but they still had to wait until they were on the verge of reaching the land of Israel. Thus, in the 40th year, when the lesson of the mahn had been instilled in them and they were on the verge of entering the land of Israel, the covenant could be completed on the plains of Moav and the people then had full covenantal knowledge of G-d.