Numbers 18:2
And thy brethren also of the tribe of Levi, the tribe of thy father, bring thou with thee, that they may be joined unto thee, and minister unto thee: but thou and thy sons with thee shall minister before the tabernacle of witness. And thy brethren also of the tribe of Levi, the tribe of thy father, bring thou with thee, that they may be joined unto thee, and minister unto thee: but thou and thy sons with thee shall minister before the tabernacle of witness.
The Levites were appointed in place of Israel's firstborn, honoring their fidelity during the apostasy over the golden calf. They were joined to Aaron and his sons to assist them in caring for the tabernacle and its sanctuary service. The other Levites cared for the tabernacle and its furnishings and attended the priests, but they could not offer sacrifices or burn incense, and they could not look upon the holy things until they were covered.
Aaron and his sons alone were to minister before the tabernacle of witness, before the Lord. These distinct duties were assigned by God and were not taken up by human choice. This divinely ordered distinction was meant to prevent jealousy and preserve proper order in sanctuary service.