What is the Pasuk telling us here?
Rashi #1 (by implication) and Seforno: When Hashem instructed Moshe to speak to Par'oh, 1 and Moshe complained that Par'oh 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 Par'oh's 'god' and Aharon, Moshe's interpreter. 4
Rashi #2 (in Bo, Sh'mos 12:2): It is teaching us that Hashem only spoke to Moshe in daytime. 5
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 to Moshe exclusively. 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 explain that it pertains to the Pesukim that follow. See Sifsei Chachamim.
See Bo, Sh'mos 12:2 - Peirush Yonasan, based on the Mechilta.