The ESV Study Bible says this about our passage in Numbers today (Chapters 3-4)
This chapter again shows preparations being made for the march into Canaan. It underlines once again the dangerous holiness of God: he cannot be approached casually. It also emphasizes the importance of the ministry of the Levites, who make possible the transport of the tabernacle to the land and who guard it from intrusion.
Knowing God through His Word ... Day by Day 