Despite their fear of the peoples around them, they built the altar on its foundation and sacrificed burnt offerings on it to the Lord, both the morning and evening sacrifices. Ezra 3:3

            Once the altar was ready, the sacrifices could begin and the Jews could continue the rituals to reconnect with God.  No burnt offering could occur without the altar.  Next they could lay the foundation and move the altar on top of it.  First things first, they wanted to show God their heart and the altar provided the way for them to do that.