Covering of the blood speaks to the atonement of sin in the sight of God. Blood is the only remedy which God has accepted since the days of Adam and Eve at the fall. The animal which God killed to fashion skins to cover their nakedness is illustrative of the eventual death of Christ; the ultimate sacrifice to come later.
Recall Cain and Able, Able made sacrifice to God with a blood/burnt offering while Cain offered fruits and vegetables of the field. While it was Cain's good intention, it was not the requirement which God had established as forgiveness of sin. Without the shedding of blood there can be no redress or redemption of sin.
We know the wages of sin are death; loss of life results for those who remain in their unrepentant state but there must be a shedding of blood to atone, cover and cleanse that sin which exists in the human race.
Good question.
