Numbers 4:27
At the appointment of Aaron and his sons shall be all the service of the sons of the Gershonites, in all their burdens, and in all their service: and ye shall appoint unto them in charge all their burdens. At the appointment of Aaron and his sons shall be all the service of the sons of the Gershonites, in all their burdens, and in all their service: and ye shall appoint unto them in charge all their burdens.
All the service of the sons of the Gershonites, in all their burdens and in all their service, was to be at the appointment of Aaron and his sons, who were to appoint unto them in charge all their burdens. Their appointed work was the care of the curtains and hangings of the tabernacle. They camped behind the tabernacle.
The work of carrying and caring for the tabernacle was allotted in an orderly way, showing that God organized the sacred service. Each group of Levites had defined work to perform in the sanctuary service, and each group was responsible for its appointed tasks.
The sacred work of transporting the tabernacle was entrusted to designated people, not left to anyone's choice nor open to anyone who sought authority for themselves. God's arrangements are to be followed rather than replaced by confusion or disobedience.