As for the sons of Merari, thou shalt number them after their families, by the house of their fathers;

Jamieson-Fausset-Brown Bible Commentary

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.

Matthew Henry's Concise Commentary

4:21-33 We have here the charge of the other two families of the Levites, which, though not so honourable as the first, yet was necessary, and to be done regularly. All the things were delivered them by name. It intimates the care God takes of his church and every member of it. The death of the saints is represented as the taking down of the tabernacle, 2Co 5:1, and the putting it off, 2Pe 1:14. All shall be raised up in the great day, when these vile bodies shall be made like the glorious body of Jesus Christ, and so shall be for ever with the Lord.