Numbers 10:21

And the Kohathites set forward, bearing the Sanctuary, the sacred furniture and the holy vessels, Numbers 7:9; and the other, the Gershonites and the Merarites, did set up the Tabernacle against they came (until they reached the place). This was the reason why these two companies of Levites preceded the tribes led by the Reubenites, Cf Numbers 2:17, since they thereby gained enough time to have the tent erected before the Kohathites came with the furniture, the altars and the holy vessels.