Numbers 18:4
And they shall be joined unto thee, and keep the charge of the tabernacle of the congregation, for all the service of the tabernacle: and a stranger shall not come nigh unto you. And they shall be joined unto thee, and keep the charge of the tabernacle of the congregation, for all the service of the tabernacle: and a stranger shall not come nigh unto you.
The Levites were joined to Aaron to keep the charge of the tabernacle of the congregation for all its service. Their assigned work was to care for the tabernacle and its furnishings and to assist the priests. The ministry before the Lord as priests was reserved to Aaron and his sons, making the Levites' assisting work distinct from the priests' work.
They were not to approach the sanctuary vessels or the altar. They were not permitted to offer sacrifice or to burn incense, nor to look upon the holy objects until those objects had been covered. No stranger or unauthorized person was to come near. These duties and their boundaries were appointed by God.