Thus you shall separate the Levites from among the people of Israel, and the Levites shall be mine. And after that the Levites shall go in to serve at the tent of meeting, when you have cleansed them and offered them as a wave offering. For they are wholly given to me from among the people of Israel. Instead of all who open the womb, the firstborn of all the people of Israel, I have taken them for myself.
The Levites enjoyed a unique position in Israel as the worship working class of the people. They were the men tasked with the physical tasks of the sacrificial system begun with the tabernacle in the wilderness. They cared for and carried the ark of the covenant and all the furnishings that God required for proper worship. They kept all the worship routines going at the tabernacle, from managing the burning of incense and lamps to the keeping of livestock brought in for blood sacrifice. They were solely devoted to the ministry of worship in Israel.
God claimed them by rights. Every firstborn was "redeemed" by the Law through a sacrifice required by God. And in his place, God substituted a Levite. All male Levites were in "replacement" for the firstborn males of the rest of Israel whom God saved in the Passoever. They were called to worship God with the nation and for the nation.
In them we have instruction on the seriousness of worshiping a holy God. God took the Levites for Himself. He demanded that Israel set them aside and that they do what every family should have done... devote 100% of their life work to the worship of the Lord so that sin could be atoned in Israel by sacrifice. It is a serious matter to get sinners right with a holy God. It required thousands of full-time men in the wilderness tabernacle to devote themselves fully to the requirements God gave in the Law. But today I am redeemed by the finished sacrifice of only one--- the only begotten Son -- Jesus Christ. He is my perfect priest. Praise Him!