The Levites were giving a special responsibility. Some suggest that the book of Numbers exempts them from military service because they were not to be included in the census of fighting men. My take on that is that the census only counted the “Offensive” forces of Israel. The Levites were given a “defensive” responsibility. They were positioned closes to the Tabernacle which was at the center of the camp. They surrounded it on all sides with Moses and Aaron on the east side at the entrance. They were the last line of defense protecting the most important structure in the camp. They were to dismantle it when the camp was about to move and carry it and all the instruments associated with worship and then erect it again at the new location.
The structure of worship was closely associated with their warfare. Worship and warfare may seem unrelated, but in God’s economy, they go together. The priests were often called to take part of military efforts. The first effort at Jericho is lead by the priests who lead the march around the city every day for seven days and on the seventh day they were to blow the trumpets as all the people shouted. One of the major themes of the Book of Revelation is God’s warfare against evil on earth and His receiving worship in heaven.
Unless the people of God are right with the Lord in their worship, they can’t face their enemies and defeat them in warfare. I love what the Psalmist sings in Psalm 149, verse six, he says, “Let the high praises of God be in their mouth, and a two-edged sword in their hand”