Mountain of the Lord
I find in this the primary usefulness of the temple. The reminder that even the flawed and irresponsible can enter into God's presence. The beautific unfairness that allows the sinner in by the suffering of the innocent. The glory of God's atonement in the creation of the world.
The purpose of temples is to have places set apart where God and man can meet. It may be hubristic of us to assume He is interested. Nonetheless experience, temple or otherwise, shows that He is.
Thank God for that!