Parashat Vayikra (Vayikra chapters 1-5) reviews the laws of the sacrifices from the point of view of the Israelites bringing those offerings. The first two chapters of Parashat Tzav (Vayikra 6-7) are laws of the sacrifices as presented to the priests. In general, the same sacrifices are discussed in both parashot. What is the reason for this division? Couldn’t all of the laws be consolidated to one parasha?
An answer to this question may be found in a midrash, quoted by Rashi on Vayikra 6:2 (the opening of Parashat Tzav), which begins “Command [tzav] Aaron and his sons thus”:
“The expression ‘Command …!’ always implies urging on to carry out a command, implying too, that it comes into force at once, and is binding upon future generations. R. Shimon said: ‘Especially Scripture must urge on the fulfillment of the commands in a case where monetary loss is involved.’”
Rabbi Shimon’s adding on the concern of “monetary loss” requires clarification. Many different explanations have been given as to how the priests were liable to lose money with these laws. I found the explanation of the Torah Temimah commentary particularly convincing. He writes that Rabbi Shimon was concerned about the priests not being careful regarding the first type of sacrifice mentioned in Parashat Tzav – the “olah” (burnt offering). Since the meat of this offering was entirely burnt as a sacrifice, with nothing left for the priests to eat (in contrast with other offerings), they may be negligent in their attention to it.
I think we can expand this idea to the general need for the laws for the priests being separated into Parashat Tzav. In many religions, not only were the priests exclusively responsible for religious rituals, the lay people didn’t even know the rules of how those rituals were performed. The Torah ensured that every person knew the laws, and therefore the book of Vayikra opened with the laws as applied to every Israelite. And then it added a section for the priests (which the Sages designated as a separate parasha), which began with an urge for additional vigilance. The priests could easily take advantage of the sacrificial system, especially when they had a possibility of monetary gain. (For example see the behavior of Hofni and Pinchas in Shmuel I 2:12-17). The Torah wanted to prevent any possibility of corruption of this nature, and so dedicated laws with details about precisely how the priests should deal with these offerings.