The reason why they gathered as one man to rebuild the temple, is to worship God. The greatest act of man is to worship God. This is our primary reason for existence. This is why God creates us. This is why God sustains us. This is why God has redeemed us. This is why God glorifies us, equips us, and provides for us, and so on. It is that we would glorify him, that we would honour him. The Shorter Westminster Catechism says,
Michael Voytekman's primary purpose is to glorify God and to enjoy him forever.I do like what John Piper says,glorify God by enjoying him; by having that relationship with him and the joy that comes with that.And so we are called and created for that relationship, that service, and that worship. He has freed us from the chains of sin, that we might worship and serve only him.1
1 When the seventh month came, and the children of Israel were in the towns, the people gathered as one man to Jerusalem.