Albert Barnes Commentary Numbers 4:23

"from thirty years old and upward until fifty years old shalt thou number them; all that enter in to wait upon the service, to do the work in the tent of meeting." — Numbers 4:23 (ASV)

enter in to perform the service - literally, as in the margin, to war the warfare, or, as the same phrase in part is rendered, (Numbers 4:3), enter into the host to do the work. The language is military. The service of God is a sacred warfare (Numbers 8:24–25 marginal reading).