Numbers 18:7
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.
The priest's office for everything of the altar and within the veil was given to Aaron and his sons and to his descendants after them as a service of gift from God. That office belonged to them alone, and they alone were permitted to minister before the Lord at the altar and within the sanctuary. The work was restricted to those God had appointed, and any unauthorized stranger who came near to serve was to be put to death.
The other Levites did not share that priestly office but were assigned to care for the tabernacle and its furnishings and to assist the priests. They could not offer sacrifices or burn incense, nor look upon the holy things before they were covered. Their appointment in place of Israel's firstborn honored their faithfulness in serving God and in carrying out judgment when Israel worshiped the golden calf.
King Uzziah's attempt to burn incense violated this command. Neither his royal position nor his record of service excused his presumption. The command teaches that each worker is to keep to the duties entrusted to him.