He says that the Torah adds this detail about Moshe because it explains why Hashem had to defend him. Moshe was too humble to stick up for himself.
Ramban then quotes the opinion of Rav Eliezer which says the same thing. That Moshe would never ask anything of anyone or do anything and that he wouldn't go against anyone.
Lastly, he quotes an opinion which is different than his own, the Sifri. It says that the Torah uses this phrase to describe Moshe because he was standing with his siblings when they spoke about him and he was humble in that he controlled himself and didn't respond. Then Hashem responded for him.
