Perhaps this altar was designed to represent God's covenant, and the twelve pillars the twelve tribes of Israel. Moses standing between the altar and the pillars, perhaps represented the Lord Jesus as the Mediator.
Related Commentaries of Exodus 24:4
Exodus 24:4
4 And Moses wrote all the words of the LORD, and rose up early in the morning, and builded an altar under the hill, and twelve pillars, according to the twelve tribes of Israel.