“Thus you shall keep the sons of Israel separated from their uncleanness, so that they will not die in their uncleanness by their defiling My tabernacle that is among them.”

Why was ritual cleanness so important that God gave the people of Israel the book of Leviticus? This verse sums it up. God’s earthly resting place, the tabernacle, was to be established, setup and maintained among God’s people, the people of Israel. As one moved closer to the proximity of God there was a greater requirement for ritual purification.

The only one would could enter the actual tabernacle was the High Priest, and that only once a year after extensive purification rituals. The only ones who could present sacrifices to God were the priests who were established as a separate class with special purification rituals. Before the people could come before the priests they needed to abide by certain purification rituals. The people as a whole needed to live by certain purification rituals.

There are concentric circles of purification starting with the people, who must maintain a cleanliness code. Then the worshipers who must maintain a stricter code. Then the priests who must maintain still a stricter code. Then the high priest who must maintain the strictest level of purification.

As God allowed people to move closer to him the need for purification was greater.