In Numbers 4:9, this signifies spiritual warfare as they minister in lieu of actual military service. (Apocalypse Revealed 500)

In Jeremiah 33:22, the ministry of the Levites signifies people in love to the Lord and its derivative worship. (Apocalypse Explained 444[7])

In Numbers 8:24, 25, their Ministry was called warfare, or military service because it represents the Lord's work of salvation in removing evils and falsities from hell which are continually fighting against goods and truths. (Apocalypse Explained 734[14])