Charles Ellicott 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)

As a gift. —Hebrew, given, as in Numbers 8:16.

That there be no plague among the children of Israel. —The appointment of the Levites in place of the firstborn was calculated to ensure the reverent and orderly discharge of the duties of the Sanctuary, and to operate as a safeguard against those sins of omission and commission into which the firstborn would have been more likely to be led, and which would have provoked the Divine wrath against the Israelites generally.