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 The lot came out for the clans of the Kohathites. So those Levites who were descendants of Aaron the priest received by lot from the tribes of Judah, Simeon, and Benjamin, thirteen cities.