The main practical reason for learning how to think well about the eternal life of the Trinity is that it is the background for the gospel. The blessedness of God's inner life is the only thing that is even better than the good news. The life of God in itself is the source of all the riches that fund the economy of salvation. It is also the guarantee that God's grace is based on his character rather than on anything outside himself.
Fred Sanders, The Deep Things of God (Crossway, 2010), page 83.