Absorbed with matters of purity and ritual, chapter 16 treats of the relationship between God and the community, including the individual. Impurities threaten to render the divine inaccessible. So once a year the pristine holiness of the sanctuary must be restored. In contrast, chapter 18 treats of the manner in which human beings ought to relate to each other. If every woman in the household is fair game for sexual conquest, if every form of sexual experimentation is within bounds, then neither the family nor society will long endure.
Sexual restraint generates cohesion, intensifying the relationships that are countenanced.
Religion must comprise morality as well as theology if it is to enhance the social compact. Ancient Israel was to distance itself from Egypt not only geographically but also spiritually in its patterns of living no less than in its conception of God. Thus the Yom Kippur liturgy appropriates both chapters 16 and 18 in order to stress the totality of Judaism as a system of belief and practice, of ritual and morality. The balance between the two is what counts. One must not be allowed to overwhelm the other.