"God did this [assumed human being] without ceasing to be God. He differentiates himself from all false Gods (among whom the god of Islam is especially characteristic in this respect) by the fact that he is not a prisoner of his own exalted status, but can also be lowly - not in the surrender but the affirmation of his divine majesty."
In other words, God does not do something un-Godlike when he assumes human nature and unites it to his own in Jesus Christ. He is not giving himself away in giving himself to us. He remains God.
"He exists even in himself as God, not only in the majesty of the Father, but also and in the same reality and Godhead as the Son begotten of the Father and following Him and ordered in accordance with Him. In itself and as such, then, humility is not alien to the nature of the true God..."
Because God is, in himself, the Son who obeys as well as the Father who commands, humility and service are not strange things to him, taken on only in the incarnation. Humility as well as majesty is proper to God.
"We can only say that in its great inconceivability - always new and surprising when we try to conceive it - this reason [that is, God's mercy] is holy and righteous because it corresponds to the humility of the Eternal Son as it takes place in the supreme reality of the intra-trinitarian life of God himself..."
So without taking the wonder out of the incarnation, we can say that it is grounded in God's being, not as something he must do of necessity, but as something which corresponds to his eternal character.
Why does this matter?
Firstly, it matters because the incarnation is revelatory. In Christ, we see God. That could not be the case if the incarnation - the humility of Christ - were basically alien to God as he really is. We would look at Christ and see something other than God, something that God has made himself - mercifully, to be sure, but not revelatory.
Secondly, it helps us to know what it means to be godlike. Imitating our God will mean humble service.