Numbers 19:1

The principle that death and all pertaining to it, as being the manifestation and result of sin Genesis 2:17, are defiling, and so lead to interruption of the living relationship between God and His people, is not now introduced for the first time, nor is it at all peculiar to the Mosaic law. It was, on the contrary, traditional among the Israelites from the earliest times, it is assumed in various enactments made already (compare Numbers 5:2; Numbers 9:6 ff; Leviticus 10:1, Leviticus 10:7; Leviticus 11:8, Leviticus 11:11, Leviticus 11:24; Leviticus 21:1 ff), and it is traceable in various forms among many nations, both ancient and modern. Moses adopted, here as elsewhere, existing and ancient customs, with significant additions, as helps in the spiritual education of his people.

The ordinance was probably given at this time because the plague which happened Numbers 16:46-50 about the matter of Korah had spread the defilement of death so widely through the camp as to seem to require some special measures of purification, more particularly as the deaths through it were in an extraordinary manner the penalty of sin.