Jamieson, Fausset and Brown on Numbers 4:29-33

29-33. As for the sons of Merari--They carried the coarser and heavier appurtenances, which, however, were so important and necessary, that an inventory was kept of them--not only on account of their number and variety, but of their comparative commonness and smallness, which might have led to their being lost or missing through carelessness, inadvertency, or neglect. It was a useful lesson, showing that God disregards nothing pertaining to His service, and that even in the least and most trivial matters, He requires the duty of faithful obedience.