What can the so-called Extra Calvinisticum do for me?

It's a question everyone's asking, I know!

In case you need a little reminder, the so-called "extra Calvinisticum" is the idea that "the Eternal Son of God, even after the Incarnation, was united to the human nature to form One Person but was not restricted to the flesh.” (Willis, p1). See e.g. Institutes 2.13.4 and 4.17.30.

If you think that God is essentially omnipresent, it seems to me you have to say that if you think that the Human Christ was fully God and that his humanity was not omnipresent.

Willis writes of the intention, role and effect of the extra Calvinisticum in Calvin's thought:

In teaching what came later to be called the “extra Calvinisticum,” Calvin the Biblical theologian endevours, with the force of his humanist learning turned to the service of Christian theology, to safeguard an essential response to revelation which the Church before him also found it necessary to protect. It is him way of taking account of the fact that in Christ God is powerfully for us. It is not intended to evade the Christ witnessed to by the apostolic message, nor is it intended to buttress an allegedly natural theology. On the contrary, the “extra Calvinisticum” emphasizes that the God at work in Jesus Christ is one and the same with the God who sustains and orders the universe. He is the Triune God, as is manifest in the prominent role assigned to the Holy Spirit in the dynamics of the Incarnate life. The humanity of Christ can develop in a special way without transgressing the bounds of genuine humanity because of the gifts which the Logos conveys to it by his Spirit. In the “extra Calvinisticum,” Calvin is asserting that Christ is able to be God for us because he does not cease to be God over us in the Incarnation and because the humanity of Christ never ceases to be our humanity in the movement of God towards us. In Jesus Christ the vindication of the majesty of God and the re-establishment and fulfilment of the humanity of man take place. It is this gracious relationship – Creator-created, Redeemer-redeemed, Sanctifier-sanctified – on which the right ordering of history depends: such is Calvin’s affirmation in the “extra Calvinisticum,” not the philosophical principle finitum non capax infiniti. The “extra Calvinisticum” is not a sign of the discontinuity between creation and redemption, but of the fact that by assuming our condition the Eternal Son did not relinquish part of his empire but extended that empire over lost ground. Election to the body of Christ means being united to Head by the Holy Spirit, and conforming to the image of Christ in the world by the Holy Spirit. The “extra Calvinisticum” provides for Christ’s Lorship over all the world and yet his special presence with and Lordship over the Church, through the Holy Spirit; and, it bestows a Christological content upon the role which the order of nature plays in ethics.

E. David Willis, Calvin’s Catholic Christology: The Function of the So-Called Extra Calvinisticum in Calvin’s Theology Studies in Medieval and Reformation Thought volume 2 (Leiden, E. J. Brill, 1966), pp6-7