The Lord instructs here the priests that the office which they fill, and the help which they enjoy, are gifts from Him, and are to be viewed as such.
Related Commentaries of Numbers 18:6
Numbers 18:6-7
6 And I, behold, I have taken your brethren the Levites from among the children of Israel: to you they are given as a gift for the LORD, to do the service of the tabernacle of the congregation.
7 Therefore thou and thy sons with thee shall keep your priest's office for every thing of the altar, and within the vail; and ye shall serve: I have given your priest's office unto you as a service of gift: and the stranger that cometh nigh shall be put to death.