Chabad.org — English translation + Rashi
Torah Temimah on the gift of the Levites to Aharon and his sons: they are “נתונים נתונים” — doubly given — for the service of the Tent and for atonement so that no plague strike Israel when approaching the Holy.
English: “I have given the Levites as a gift to Aharon and to his sons … to perform the service of the children of Israel in the Tent of Meeting and to atone for the children of Israel, so that there be no plague among the children of Israel when the children of Israel approach the Holy.”
Purpose clause — the double function (service + atonement/protection) explains why the Levites stand between Israel and the sanctuary.
The Levites are not merely assistants; they are a protective buffer that allows Israel to draw near without incurring the danger of unmediated approach.