C. F. Keil and Franz Delitzsch on Numbers 18:2-4

Num 18:2-4

Aaron was also to bring his (other) brethren (sc., to the sanctuary), viz., the tribe of Levi, that is to say, the Gershonites and Merarites, that they might attach themselves to him and serve him, both him (ואתּה) and his sons, before the tent of testimony, and discharge the duties that were binding upon them, according to Num 4:24., Num 4:31. (cf. Num 3:6-7; Num 8:26). Only they were not to come near to the holy vessels and the altar, for that would bring death both upon them and the priests (see at Num 4:15). On Num 18:4, cf. Num 1:53 and Num 3:7.