Why does the Torah mention the names of the princes of the tribes, having already listed them in Bamidbar?
Ramban: It is to teach us that, even when Yisrael traveled, the Nasi went at the head of the tribe, 1 so that they should not be like sheep without a shepherd, or to teach us that he was not replaced by a deputy. 2
Ramban: It cannot be coming to teach us that the Nesi'im were all still alive, since the Torah already taught us that nobody died between the time that they were counted and the time they traveled (Refer to 2:4:1:1*).
Ramban: Like we find the Mashi'ach Milchamah, who deputized for the Kohen Gadol when they went to war.