John Calvin Commentary Numbers 8:19

"And I have given the Levites as a gift to Aaron and to his sons from among the children of Israel, to do the service of the children of Israel in the tent of meeting, and to make atonement for the children of Israel; that there be no plague among the children of Israel, when the children of Israel come nigh unto the sanctuary." — Numbers 8:19 (ASV)

And I have given the Levites. He declares on what terms He desires to have them as His own, namely, that they may be directed by Aaron and obey his commands. For “a gift” is not to be understood as an act by which a person resigns and cedes his own right to another; rather, when He devotes them to the ministry of the sanctuary, He desires that they should have a leader and master.

At the end of the verse, Moses teaches that this is done for the advantage and benefit of the whole people; from this it follows that there was no room for ill-will towards them, unless the people might perhaps be annoyed that God had taken measures for their welfare.

A twofold advantage is pointed out: first, because they were to be the intercessors or ministers of reconciliation (for either sense would be appropriate); and secondly, because, while they would be the guardians of the sanctuary, they would prevent the Israelites from bringing destruction upon themselves by their rash approach to it.