Albert Barnes Commentary Numbers 4:28-33

"This is the service of the families of the sons of the Gershonites in the tent of meeting: and their charge shall be under the hand of Ithamar the son of Aaron the priest. As for the sons of Merari, thou shalt number them by their families, by their fathers` houses; from thirty years old and upward even unto fifty years old shalt thou number them, every one that entereth upon the service, to do the work of the tent of meeting. And this is the charge of their burden, according to all their service in the tent of meeting: the boards of the tabernacle, and the bars thereof, and the pillars thereof, and the sockets thereof, and the pillars of the court round about, and their sockets, and their pins, and their cords, with all their instruments, and with all their service: and by name ye shall appoint the instruments of the charge of their burden. This is the service of the families of the sons of Merari, according to all their service, in the tent of meeting, under the hand of Ithamar the son of Aaron the priest." — Numbers 4:28-33 (ASV)

The Gershonites and Merarites were superintended by Ithamar, Aaron’s younger son, who had already had the oversight of the tabernacle in its construction (Exodus 38:21). Thus, the permanent offices of the leaders of the Israelite community readily sprang from the duties that, under the emergencies of the first year of the Exodus, they had been led, from time to time, to undertake.

Regarding Numbers 4:32: By name you shall reckon the instruments—Or, assign them to their bearers individually, and “by name.” These “instruments” comprised the heavier parts of the tabernacle, and the order seems intended to prevent individual Merarites choosing their own burden and so throwing more than their proper share on others.