Charles Ellicott Commentary Numbers 8:15

"And after that shall the Levites go in to do the service of the tent of meeting: and thou shalt cleanse them, and offer them for a wave-offering." — Numbers 8:15 (ASV)

And after that the Levites shall go in ... — that is, into the court of the Tabernacle to keep watch there, and to assist the priests at the altar of burnt-offering, and to take down and set up the Tabernacle as occasion might require.