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 Alors la colère de l'Eternel s'enflamma contre Moïse, et il dit: N'y a-t-il pas ton frère Aaron, le Lévite? Je sais qu'il parlera facilement. Le voici lui-même, qui vient au-devant de toi; et, quand il te verra, il se réjouira dans son cœur.