Chabad.org — English translation + Rashi
Torah Temimah emphasizes the protective function: the Levites’ presence prevents “נגף” when Israel approaches the Holy.
English: “So that there be no plague among the children of Israel when the children of Israel approach the Holy” — the Levites’ appointed service is the mechanism that averts that danger.
Protective mediation — the verse explicitly links the Levites’ role to the prevention of catastrophic consequence.
Approach to the sacred requires an appointed intermediary class; without them the same approach would be fatal.