Matthew Poole and continuators on Numbers 3:38

38 But those that encamp before the tabernacle toward the east, even before the tabernacle of the congregation eastward; shall be Moses and Aaron, and his sons, keeping the charge of the sanctuary for the charge of the children of Israel : and the stranger that cometh nigh shall be put to death.

Either in their stead, that charge which they were obliged to keep either by themselves or by others appointed by them, if God had not committed it to those: or for their good and service and benefit, for their preservation, as the word may be rendred.