Because the incarnation is such a unique event in history – and also in other religions, to be honest. Not other religion has a doctrine of incarnation like Christianity. Hercules as a God-man is very different to Jesus as the God-man – it’s not hard to loose our way and go off the reservation and loose touch with the Bible. Check out this post to see some of the options. And so we have doctrines that help us preserve and protect a Biblical position. And one of those doctrines is the anhypostasis and enhypostasis. Both thoughts are very important to maintaining Chalcedonian orthodoxy, and both really belong together.
When we’re talking an- or en- we’re talking about the humanity of Jesus. What we’re safeguarding is that there is only one person in Jesus Christ. This one person means that the humanity of Jesus wasn’t a human person before the incarnation. The incarnation is not a uniting of a human with the Son of God. In other words his human nature had no independent hypostasis apart from the Word. If there was a human person to whom the person of the Son was added there would then have been two persons in the incarnate Jesus. But the human nature of Jesus never existed apart from the incarnation of the Son. That is, it is anhypostasia, without person.
From the first moment of the human nature’s existence it had it’s personal subsistence in the person of the Son. The personhood of that human nature was provided by the second person of the Trinity. This is the meaning of enhypostasia. The enhypostasia preserves the full humanity of Jesus as a genuine historical human person.
The anhypostasia stresses the general humanity taken up by the Word. That he is a human in solidarity with all humans. He shares humanity with us. The enhypostasia stresses the particular humanity of the one man Jesus, whose person is the very person of the divine Son himself.
The two ideas belong together and the two ideas flow from one into the other. You move from the anhypostasia on almost effortlessly to the enhypostasia. And so then from the enhypostasis we have to then go back to the anhypostasis and say that while the Son does take on our real, shared human nature and really is like us, at the same time his full and complete human nature was united to God in a unique way as ours is not and never will be, that is hypostatically in one person. Our union with God will always be a union of two people. The unity of God and man in Jesus is a hypostatic union in that it takes place within, so to speak, one person.
Therefore, at the same time as Jesus is like us he is also very unlike us. Not that he’s unlike us in terms of his human nature, his human nature is totally human. He is unlike us in the way that his human nature is united to the divine nature in the one person of the Son.
And so the anhypostasis and the enhypostasis are really one idea, and they belong together, in asserting and safeguarding the real humanity of Jesus. He came in solidarity with our common human nature, anhypostatically, and within that solidarity he came as an individual human being in solidarity with our personal relations and responsibilities and encounters and so entered as deeply as can be into personal relationship with sinners.
The real humanity of Jesus ensures that God has really come to us. He has come all the way to us in love. The humanity of Jesus signifies that God has really come and is really present. If he’s not really human then God has not come all the way to us. But because Jesus is totally human it guarantees for us that we really have God among us. And so revelation is secured and guaranteed because it ensures for us that the eternal God can be really known within temporal and contingent human existence. The humanity of Jesus ensures that revelation is both real and accessible to us.
So to summarise: the anhypostasis maintains that God has become one of US. The enhypostasis maintains that GOD has become one of us.