"When Moses had proclaimed every commandment of the law to the people, he took the blood of calves, together with water, scarlet wool and branches of hyssop, and sprinkled the scroll and all the people. He said, 'This is the blood of the covenant, which God has commanded you to keep.' In the same way, he sprinkled with the blood both the tabernacle and everything used in its ceremonies. In fact, the law requires that nearly everything be cleansed with blood, and without the shedding of blood there is no forgiveness." NIV translation
In the same way, covenants under the law required the shedding of blood. Like a will, this blood was required for the contract to take effect.