Numbers 18:6
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. 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.
God took the Levites from among the children of Israel and gave them as a gift for the Lord to Aaron and his sons to do the service of the tabernacle of the congregation. They were set apart from the rest of Israel to work together with the priests in caring for the tabernacle. In this tabernacle work they took the place of Israel's firstborn. Their appointment honored their faithfulness to God, shown when they stood for His service at the time Israel worshiped the golden calf.
Their work was to assist the priests and to care for the tabernacle and its furnishings, remaining within the duties and limits God assigned to them. Only Aaron's family could serve as priests, while the other Levites were barred from offering sacrifices or burning incense. They were also forbidden to look upon the holy things before they were covered. These appointments were made by God Himself, not by men seeking prominence for themselves.