God takes the Levites instead of the first-born of the Israelites. He claims the first-born this way in the stead of the first born from the Passover. They are His and will serve Him.
They are to care for the tabernacle and God’s materials for it. In this way, as the other people are numbered based on who is able to go to war, they are to “battle” for the people in their care for God’s instructions.
Again, the handling and moving of the materials have detailed and specific instructions for the Levites to follow. This means take it seriously.