The priesthood received a portion of some of the offerings, the devoted things that Israel brought to be offered to God. However, they could only eat it if they themselves were clean.
It is important for us to understand that the defiling thing (what makes a person unclean) may not, of itself, be sin—or that the person has necessarily sinned by coming in contact with it. What it represents or symbolizes is important to us here. We are dealing with a ritual, so the defiling thing represents, symbolizes, typifies, sin and its effects. This is not to say that coming into contact with some of these things might not be potentially physically harmful—because disease may very well be communicated by coming into contact with a corpse, for example. Nonetheless, the possibility of defilement by sin is taught in every case in which God declares a person unclean. That person is thus unsuited to serve Him until the defilement is removed through the washing ceremonies He prescribes.
Why is God so insistent about avoiding contamination? The answer appears in Haggai's question to the priests:
On the twenty-fourth day of the ninth month, in the second year of Darius, the word of the LORD came by Haggai the prophet, saying, "Thus says the LORD of hosts: 'Now, ask the priests concerning the law, saying, "If one carries holy meat in the fold of his garment, and with the edge he touches bread or stew, wine or oil, or any food, will it become holy?"'" Then the priests answered and said, "No." And Haggai said, "If one who is unclean because of a dead body touches any of these, will it be unclean?" So the priests answered and said, "It shall be unclean." Then Haggai answered and said, "'So is this people, and so is this nation before Me,' says the LORD, 'and so is every work of their hands; and what they offer there is unclean.'" (Haggai 2:10-14)
God is consistent about avoiding contamination because it is so easy for what is contaminating and defiling to our character to transfer to us. Human nature is like a magnet that attracts defilement. Above all people in Israel, the priests had to be clean, and the principle applies to us. Theirs was mostly a physical cleanliness, but moral, spiritual, and ethical cleanliness is certainly implied. By contrast, ours is primarily a spiritual, moral, and ethical cleanliness with the physical implied.
These Old Covenant laws serve as reminders and guides about how serious God is—and therefore, we should be—about not becoming contaminated by the world.