This is not mentioned to reveal to Moses that Aaron is of the tribe of Levi, because Aaron is his brother, so he knows that. Later we read of the Levites, who are set apart because they did not share in the worship of the golden calve (Exodus 32:25–29). They have to care for the tabernacle. The sons of Aaron will be priests to intermediate for the people to the Lord (atonement by sacrifices), but also to “teach the people of Israel all the statutes that the Lord has spoken to them by Moses” (Leviticus 10:11). God can call Aaron “the Levite” proleptic: it points to the future call of the Levites
14 Then the anger of the LORD was kindled against Moses and he said, “Is there not Aaron, your brother, the Levite? I know that he can speak well. Behold, he is coming out to meet you, and when he sees you, he will be glad in his heart.