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.

And the Lord spake unto Moses, and unto Aaron,.... Not at this time, after the business of the spies, and the affair of Korah, but before the children of Israel departed from Sinai; and so Aben Ezra observes, that this was spoken in the wilderness of Sinai, when the Lord commanded to put unclean persons out of the camp, and when some were defiled with a dead body, and unfit for the passover, Numbers 5:2; and mention is made of the "water of purifying", Numbers 8:7,
saying; as follows.

The heifer was to be wholly burned. This typified the painful sufferings of our Lord Jesus, both in soul and body, as a sacrifice made by fire, to satisfy God's justice for man's sin. These ashes are said to be laid up as a purification for sin, because, though they were only to purify from ceremonial uncleanness, yet they were a type of that purification for sin which our Lord Jesus made by his death. The blood of Christ is laid up for us in the word and sacraments, as a fountain of merit, to which by faith we may have constant recourse, for cleansing our consciences.

In order that a consciousness of the continuance of the covenant relation might be kept alive during the dying out of the race that had fallen under the judgment of God, after the severe stroke with which the Lord had visited the whole nation in consequence of the rebellion of the company of Korah, He gave the law concerning purification from the uncleanness of death, in which first of all the preparation of a sprinkling water is commanded for the removal of this uncleanness (Numbers 19:1-10); and then, secondly, the use of this purifying water enjoined as an eternal statute (Numbers 19:10-22). The thought that death, and the putrefaction of death, as being the embodiment of sin, defiled and excluded from fellowship with the holy God, was a view of the fall and its consequences which had been handed down from the primeval age, and which was not only shared by the Israelites with many of the nations of antiquity,
(Note: Vid., Bhr, Symbolik, ii. pp. 466ff.; Sommer, bibl. Abhdll. pp. 271ff.; Knobel on this chapter, and Leyrer in Herzog's Cyclopaedia.)
but presupposed by the laws given on Sinai as a truth well known in Israel; and at the same time confirmed, both in the prohibition of the priests from defiling themselves with the dead, except in the case of their nearest blood-relations (Leviticus 21:1-6, Leviticus 21:10-12), and in the command, that every one who was defiled by a corpse should be removed out of the camp (Numbers 5:2-4). Now, so long as the mortality within the congregation did not exceed the natural limits, the traditional modes of purification would be quite sufficient. But when it prevailed to a hitherto unheard-of extent, in consequence of the sentence pronounced by God, the defilements would necessarily be so crowded together, that the whole congregation would be in danger of being infected with the defilement of death, and of forfeiting its vocation to be the holy nation of Jehovah, unless God provided it with the means of cleansing itself from this uncleanness, without losing the fellowship of His covenant of grace. The law which follows furnished the means. In Numbers 19:2 this law is called התּורה חקּת, a "statute of instruction," or law-statute. This combination of the two words commonly used for law and statute, which is only met with again in Numbers 31:21, and there, as here, in connection with a rule relating to purification from the uncleanness of death, is probably intended to give emphasis to the design of the law about to be given, to point it out as one of great importance, but not as decretum absque ulla ratione, a decree without any reason, as the Rabbins suppose.