Jamieson, Fausset and Brown on Numbers 4:4-15

4-15. This shall be the service of the sons of Kohath, &c.--They are mentioned first, from their close connection with Aaron; and the special department of duty assigned to them during the journeyings of Israel accorded with the charge they had received of the precious contents of the tabernacle. But these were to be previously covered by the common priests, who, as well as the high priest, were admitted on such necessary occasions into the holy place. This was an exception to the general rule, which prohibited the entrance of any but the high priest. But when the cloud removed from the tabernacle, the sanctuary might be entered by the common priests, as to them was reserved the exclusive privilege of packing the sacred utensils; and it was not till the holy things were thus ready for carriage, that the Kohathites were allowed to approach.