Charles Ellicott Commentary Numbers 3:9

"And thou shalt give the Levites unto Aaron and to his sons: they are wholly given unto him on the behalf of the children of Israel." — Numbers 3:9 (ASV)

They are wholly given to him. —Hebrew, Given, given are they to him. This repetition of the word nethunim (given) is emphatic. The same repetition occurs in Numbers 8:16, where the Levites are represented as wholly given to the Lord instead of the firstborn; and in Numbers 3:19 of that chapter, as in Numbers 3:12 of this chapter, they are represented as being given by Him to Aaron and his sons. The word nethunim must not be confounded with Nethinim, the name given at a later date (1 Chronicles 9:2; Ezra 2:43; Nehemiah 3:26; Nehemiah 3:31) to the Gibeonites, who were made hewers of wood and drawers of water for the congregation and for the altar of the Lord (Joshua 9:27).

The tribe of Levi had proved themselves the most zealous for the honor of the Lord at the time of the worship of the golden calf (Exodus 32:26–29), and it was then that Moses gave them the charge to consecrate themselves (literally, to fill their hands, compare to Numbers 3:3 of this chapter) to the Lord. There was, therefore, a special reason for the selection of this tribe, independently of the fact that Moses and Aaron (and consequently the priests, as the descendants of Aaron) belonged to it.