A weapon
(actually, a missile
; see note on Nehemiah 4:17) can be used to attack an approaching enemy from a distance. A sword is functional only when the enemy has penetrated the barrage of missiles. A defensive army needs both long-range and short-range weapons. Ideally, every soldier carries the means to function in both capacities. Because of the need to keep building, Nehemiah distributed the weapons in such a way that the burden bearers could step up first to repel the invader (they also laboured somewhat away from the wall), and the builders (at the wall) could step up later.
18 And each of the builders had his sword strapped at his side while he built. The man who sounded the trumpet was beside me.