Only Hashem knows what we are thinking, so by repenting for sins that no mortal can know about, we are showing that we truly believe in Hashem. Rashi explains that this word is used to connote urgency, both for that time and for future generations. Rashi also quo tes Rabbi Shimon. Rabbi Shimon explains that the Torah uses the word tzav because the kohanim incurred a monetary loss while offering the sacrifices. The Torah wants to strengthen the kohanim in performing their service, because they might not otherwise b e inclined to serve appropriately in the face of possible financial loss.
Many commentators argue about how the monetary loss manifests itself in the kohen's life.
Some say that the kohanim would have to remain awake the entire night. This would preclude them from being able to work at their normal occupations during the day. T hey might even have to hire others to work in their stead. Others explain that the monetary loss refers to the fact that a fire was to be kept constantly burning on the altar, even at the times when there were no sacrifices. These sacrifices were offered as atonement for sins.