Jamieson, Fausset and Brown on Numbers 19:11-22

11-22. He that toucheth the dead body of any man shall be unclean--This law is noticed here to show the uses to which the water of separation [Nu 19:9] was applied. The case of a death is one; and as in every family which sustained a bereavement the members of the household became defiled, so in an immense population, where instances of mortality and other cases of uncleanness would be daily occurring, the water of separation must have been in constant requisition. To afford the necessary supply of the cleansing mixture, the Jewish writers say that a red heifer was sacrificed every year, and that the ashes, mingled with the sprinkling ingredients, were distributed through all the cities and towns of Israel.