The camp of Israel is in order and God is about to appoint certain families for certain work. But a sad story is told first. Not without a reason.
V.4 The priest was to take fire from the altar and add incense to present it to the Lord (Lev. 16:12-13). Nadab and Abihu came to the right place, and they were sons of Aaron the priest, but they took fire from some other place than from off the altar, and God killed them by fire (Lev. 10:1-2). This would remind us that God is not pleased with any pretense to worship Him unless it begins at the cross of our Lord Jesus Christ and the blood which He shed there.
V.12-13 When God killed the firstborn of Egypt, He wanted all the firstborn (males Ex. 13:12), all the tribes of Israel, as His own. Here He says He will take the tribe of Levi instead of those firstborn. All the Levites belonged specially to the Lord, and were privileged and responsible to serve Him.
V.17 The specially-honored tribe of Levi is now divided into three families — Gershon, Kohath and Merari. Each of these families has its own service, specially chosen by the Lord. We have in these early chapters of Numbers “warriors (soldiers), worshippers, and workers.” We have seen (1) a nation of warriors, gathered around their standards; put on the whole armor of God and stand for the truth; (2) a tribe of workers, the tribe of Levi; and now (3) A family of worshippers, the family of Aaron. We should surely delight to worship Him exactly as He has told us and should gladly serve the Lord however He may direct us.