blood is the "essence" of life
when the body contains life, the blood also has the life itself.
when sacrifices are made, the evils spirits absorbs the energy from the soul of the person that died.
because a body without a souls is just a body, if you ever see a dead body on intensive care unit, maintained by apparel, you will notice something strange, even with oxygen, water,all that is possible with technology, you will notice that the body is not alive, at first you will find strange then you find that is dead, because the life was gone, and life is energy.
these sacrifices are means of splashing vital energy so that the evil spirits can obtain it, since evil spirits, are descendants of the giants and the daughters and sons of the angels, aka nephilins, because God didn´t create them, the angels sinned without God permission, these beings died, but their spirits remained at earth, since they can go to heaven because they have no soul.
Dont mistake soul and spirit.
These spirits crave soul energy because they can´t obtain it, God forbid them, they are not creations of God. since they are nephilins and descended from the fallen angels of genesis 6. they have some knowledge about the heavens and earth, that has been passed to them by the fallen angels, and they use this knowledge to buy souls with sacrifice in exchange for power or knowledge of the ocult(that was not made to be know by man).