Jamieson, Fausset and Brown on Numbers 1:47-54

47-54. But the Levites ... were not numbered among them--They were obliged to keep a register of their own. They were consecrated to the priestly office, which in all countries has been exempted customarily, and in Israel by the express authority of God, from military service. The custody of the things devoted to the divine service was assigned to them so exclusively, that "no stranger"--that is, no person, not even an Israelite of any other tribe, was allowed, under penalty of death, to approach these [Nu 16:40]. Hence they encamped round the tabernacle in order that there should be no manifestation of the divine displeasure among the people. Thus the numbering of the people was subservient to the separation of the Levites from those Israelites who were fit for military service, and to the practical introduction of the law respecting the first-born, for whom the tribe of Levi became a substitute [Ex 13:2; Nu 3:12].