What is the Pasuk telling us here?
Rashi #1 (by implication) and Seforno: When Hashem instructed Moshe to speak to Pharaoh, 1 and Moshe complained that Pharaoh would not listen to him, 2 at which point Hashem instructed Aharon to accompany Moshe, 3 it was not Hashem's intention that Moshe and Aharon should act in the same capacity, but that Moshe should be Pharaoh's 'god' and Aharon, his interpreter. 4
Ramban: With reference to the previous Pesukim, 6 the Torah is informing us that although Moshe and Aharon were both responsible to take Yisrael out of Egypt, Hashem issued the instructions exclusively to Moshe. 7
Targum Yonasan: Since Hashem spoke to Moshe exclusively, it emerges that Aharon, who was equally obligated to speak to the people, 8 listened carefully to everything that Hashem commanded Moshe.
Unlike all other prophets, to whom Hashem spoke in a vision by night (See K'li Yakar).
As opposed to Rashi and Seforno, who explains that it pertains to the Pesukim that follow (See Sifsei Chachamim).