In this chapter, the Levites are counted again, but this time for the purpose of their religious duties to the tabernacle.
The Levitical census of the last chapter is most related to the redemption of the firstborn sons, and hence why it counted from one month and up. This chapter is specifically counting for the duties of the tabernacle, and hence why it only counts the males between 30 and 50, the years of service for the Levites. This census also shows that only ~8,500 of the 22,000 Levite men are within the age of service for the tabernacle, which is something that I briefly mentioned in my commentary on Num 3.
The age restriction is another reason why I said the service of the Levites is semi-professional, because they don't even start tabernacle service until they are 30, so all those earlier years are freed up for herding and farming, and probably some measure of training and education. Incidentally, this also gives us a rough idea for the life expectancy in this era, as we can presume that most Levites would normally live to at least 30, and often up to retirement at age 50. Though it's hard to say how much they might have lived past 50, we can reasonably guess that many Levites did reach age 50.
This chapter gives us a much more detailed look at the role of the Levites and how to take down and carry the pieces of the tabernacle. From it we can see that probably the most sensitive role is that of the Kohathites (the family of Aaron and Moses), because they are responsible for carrying the "most holy things" (v. 4), the furnishings of the tabernacle. We can see that the priests are responsible for covering the furnishings with blue/purple/red cloth (to protect the surfaces from scratches and scrapes) and porpoise skin (to protect them from the elements and rain). The Kohathites, then, have the ironic role of carrying a bunch of packages that they can never see in their whole lives, on pain of death. They carry these things using the golden poles that were constructed back in Exodus, which I mentioned at the time foreshadowed the transient nature of the tabernacle, that everything seemed to be built with portage in mind. One significant regulation that shows up later in the OT is that all of the holy objects must be carried on the shoulders of the Levites and not on a cart. The holy objects are both lighter and more fragile than the tabernacle structure itself, so that renders it both possible and important to carry them on human shoulders.
The separation of duties between the priests and the Levites is why I call the Levites quasi-priests. We see this separation very clearly in how the priests are assigned to go in and cover up the holy things before the Levites are allowed to enter and carry them out (though from the text it's not clear if the Levites enter the holy place to carry them out or if the priests carry them to outside the holy place for the Levites to pick them up). The Levites are given an elevated status with respect to the tabernacle because they are authorized to camp near it and to carry pieces of the tabernacle. However, they still have less sanctity than the priests who are authorized to enter the tabernacle and administrate offerings and the various rituals.
Generally, the priests are much fewer in number and handle more sensitive tasks, while the Levites are much greater in number and handle the brute force tasks of disassembling and carrying the tabernacle. The high priest is a single man and he is chiefly responsible for the core festivals like the Day of Atonement, so he sits at the pinnacle of this religious hierarchy.
For the Gershonites and the Merarites, note that the priests do not need to cover the curtains and the boards of the tabernacle with porpoise skin because 1) the tabernacle is already covered with porpoise skin, 2) more significantly, these parts of the tabernacle are not as fragile or holy as the holy things that go within. In fact, the screen of the courtyard and the outside curtains of the tabernacle are visible from outside all the time, so it doesn't make sense to cover them for transportation.
Similarly, the Gershonites and Merarites are not required to carry anything by hand, but rather they are allowed to use carts or wagons to move the pieces of the tabernacle. Both families are under the direction of Ithamar, while the Kohathites are supervised by the elder brother and next high priest, Eleazar.
This chapter ends the first series of censuses (I really think this word should be censi). There will be one more census later on, but for now we will progress to yet another priestly legal section.