For the life of the flesh is in the blood: and I have given it to you upon the altar to make an atonement for your souls: for it is the blood that maketh an atonement for the soul.
Leviticus 17:11 KJV commentary
https://bible.com/bible/1/lev.17.11.KJV
So immédiatement God sets à law in motion. Blood gives life and also I set in motion an atonement ability for the blood.
So now blood not only gives life to the body of the animal but now possesses substitution powers.
Even though at this time this is more of a revelation because blood sacrifices have been made way before the lawhttps://bible.com/bible/1/lev.16.4.KJV