Put the Levites in charge of the Tabernacle and of the Testimony, as well as all its furniture and everything it contains. They are the ones responsible for carrying the Tabernacle and all its items. They are to care for it, and to make their camp around it.
It was in response to the Lord's instructions that they were registered by Moses. Each of those registered were told what to do and what to carry, as the Lord had ordered Moses.