The Levites Given As a Gift to Aaron, Yahweh's Representative, To Do the Service of the Tent of Meeting (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 to the sanctuary.”
And having taken the Levites He has given them as a gift to Aaron and his sons. This was done for two reasons. Firstly that they might do the service in the Tent of meeting which otherwise the firstborn of Israel would have to do. And secondly in order that, by replacing the firstborn, they might make atonement for the children of Israel so that when the people of Israel approached Yahweh, they would not have to be punished with the plague because their firstborn had not been given to the Sanctuary.