With the passing of Nadab and Abihu, Aaron is left with two sons. One of those two sons is going to become the next High Priest, and *spoiler alert* it’ll be Eleazar.
That’s the reason why his name is inserted somewhat randomly in the context of the Kohathites in Numbers 4:16. In the middle of God’s instructions about covering the holy items for transport, the Text stops to mention that all of the work of the Kohathites will be done under Eleazar’s leadership and supervision.
The reason for this is both spiritual and practical.
Spiritually speaking, the actual service of the Tabernacle will fall under Eleazar’s (eventual) leadership. Ensuring the items are moved properly is a sort-of training ground for the one who will eventually become High Priest. If he can move them with reverence, he should also handle the worship with reverence.
The other reason is spiritual. As the high priest, he’s the only one that can interact with the Ark of the Covenant on any level, so he has to be involved in order to ensure everyone remains safe in the presence of God.
Specifically, God outlines a few things to Eleazar’s personal care: the oil, the incense, the grain offering, and the anointing oil. Whereas he has the oversight of the Kohathites for the rest of it, these stay with him.
Why are they given to him? Because pieces of the actual worship still need to continue, even if they’re moving. The lamps still need to be lit (Exodus 27:20), the daily meat offering still needs to be conducted (Exodus 29:39-40), and the incense needs to be burnt (Exodus 30:36). God’s worship must be continual.
I can only imagine how much of a pride-point this might be for someone like Eleazar. Even though he’s not yet the high priest, he still gets to boss around a bunch of his family members—many of whom might be significantly older and more experienced than him.
But it’s a lesson for both parties. Eleazar is learning first-hand just how much care needs to be taken into the minutiae of God’s service, and (hopefully) he’s learning that he serves only by God’s design, not his own.
The Kohathites, on the other hand, are learning to respect the institution, not the man himself, just as the Merarites and Gershonites will learn with Ithamar (Numbers 4:28, 33). Every man has a role. Every job has instructions.
It might be hard to read about this type of worship in light of New Testament worship that talks simply about singing and praying and “drawing near” to God, but the same lesson applies to us. Service, humility, reverence, understanding of roles—it all goes into what constitutes genuine worship to God.