Numbers 3:12
And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine; And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;
God took the tribe of Levi from among Israel instead of all the firstborn and claimed the Levites for Himself, setting them apart for service in the sanctuary and tabernacle. The priestly office formerly associated with the firstborn son of each family was given to Aaron and his house. The Levites were appointed to serve in the tabernacle and assist the priests, but only Aaron and his sons could minister as priests before the Lord, since the Levites' appointment did not give them the priesthood.
Choosing the Levites honored them for remaining faithful to God when Israel worshiped the golden calf. It honored them for carrying out His judgments at that time. Korah's dissatisfaction with his assigned Levite service led him to seek the higher priestly office.