The Ramban seems to be pointing out a very important lesson in relationships. The Medrish says everyone knew Moshe would pass away after revenging Midian. The Baal HaTurim says Moshe’s death was contingent on the revenge of Midian because he himself did not act zealously to stop the episode with Zimri the head of the Tribe of Shimon and Cozbi the Midianite Princess which subsequesntly resulted in 24,000 Jews dying. The events of revenging Midian and splitting the land of Sichon and Og amongst the two and a half tribes of Reuvain, Gad and half of Menashe were pretty much the last in Moshe’s life. He realized that he had to part ways from his people and that it would not be so easy for them to part. At the moment of that realization, he felt that in order to ease the process of saying goodbye it would be appropriate to mention all the good times, the positive events they went through together in the desert. This would make closure easier for everyone.
This lesson is true in all situations in when one will be leaving and not seeing someone for a while; not only in the event of death, and not only with a leader and his followers – but if one visits family and then must go back home or visits friends who they won’t see again for a long while, it is hard to say goodbye when the trip is over, or the visit is done. In order to make it easier for everyone it would seem appropriate to recap the events of the trip and remind each other about all the good times that were had.
The fact the Ramban points this out seems to indicate that it is not just a nice or cute thing to do, but is mentchlikeit, or the derech eretz way to act. This means that proper manners dictate that one should be sensitive towards the feelings of others, and try to ease the sad feeling of leaving as best as possible.