Sforno on Numbers 16:9/ספורנו על במדבר ט״ז:ט׳

1ולעמוד לפני העדה לשרתם, to serve the Lord by representing the whole community by carrying the sacred vessels, and parts of the Tabernacle. G’d selected you to perform this service לפני העדה, in full view of the whole community to serve as a reminder that another group of your nation, the firstborn, had already been deprived of this privilege due to their having participated in the sin of the golden calf, and you had been chosen to replace them.