When Moses assembled the people before him, gathered blood in a basin and flung the blood out over the people, he knew what he was doing. He knew that every morning when the people put on their clothing the bloodstains would remind them. Of the promise they had made to God with their neighbours as witnesses. On this occasion they had pledged their heartfelt, grateful service to God. But pledges and promises are easy to forget.
Dedication dribbles away.
For this reason Moses assembled the people before him for a service of re dedication, re consecration. The service was graphic. Half of the blood was thrown against the altar, the altar being the symbol of God's presence among his people. The other half of the blood was thrown over the people, the sign that they owned the sacrifice that admitted them to God's presence and allowed them to survive in God's presence. Those people never got the bloodstains out of their clothing.