Aaron (and Moses) were descended from Kohath (Exodus 6:16–20). A distinction is made between the descendants of Aaron and the other Kohathites because only Aaron and his sons were to serve as priests (Exodus 28:1 – 29:46; Numbers 3:10; Numbers 18:1–32). They also received their allotment first because of this distinction and their importance compared to the other Levites.1
The fact that they received cities from Judah, Benjamin, and Simeon was either a confirmation of their importance (since they received cities in Judah which was the most important tribe) or due to God’s foresight. The tabernacle/Tent of Meeting would later be moved to Jerusalem and with the descendants of Aaron living in that area it would be easier for them to go and minister at the temple.
4 On tira le sort pour les familles des Kehathites; et les Lévites, fils du sacrificateur Aaron, eurent par le sort treize villes de la tribu de Juda, de la tribu de Siméon et de la tribu de Benjamin;