In the Middle Ages, theologians would debate the question, Whether God would have incarnated had humanity never sinned? Different thinkers gave different answers: Thomas Aquinas for instance thought that he wouldn't, whereas John Duns Scotus thought that he would.
It seems to me that we might come across some biblical answers to this question in the affirmative. Consider, for instance, what Paul says in his letter to the Colossians. Speaking of the incarnate Christ, he says:
all things have been created through him and for him (Col 1.16).
Now if all things were created for the incarnate Christ, what else could it mean except that the goal of God from the beginning was to incarnate and live among men and women? Presumably Adam and Eve might not have sinned. But certainly there could not be an incarnate Christ without an incarnation, and if everything is created for the sake of this incarnate person, then it would seem that the incarnation was in the mind of God from the very beginning.
We get hints at this, I think, in what we read in Gen 3.8: They heard the sound of the LORD God walking in the garden at the time of the evening breeze. Here it would appear that God assumed some sort of physical state for the sake of being with the man and the woman in the garden. Perhaps we have here a mysterious and obscure hint at the Incarnation, preceding it by thousands of years. Prior to Christ's coming, what this meant would have been a mystery indeed; but perhaps now, in light of Christ's coming, we can see what this 'walking in the garden' means.
But we have to consider the question: if humanity had never sinned, what would be the purpose of God's incarnating? Why would God do it, if there weren't any sins that needed atoning and which could only be atoned by God's Son?
The answer, it seems to me, is this: because God loves us, and wants to enjoy fellowship with us in a way that could only be had through Incarnation. It seems to me obvious that the Incarnation makes a difference for divine-human relations: it is one thing to relate with God through the indwelling of the Holy Spirit, through prayer, through sacrifice and prophetic word, and so on; but it is another thing altogether to relate with God in the up-close-and-personal way we relate to other human persons. There is a level of intimacy and closeness that is not possible in any other way.
We're material beings, and our mode of fellowship is materially mediated: we come into fellowship with each other through spatial proximity and the five senses. Because we are in this way material beings, this means that the tightness of fellowship must be materially conceived. 'Out of sight, out of mind' is a proverb because it's true: we lack communion and concern for those who are distant and removed. Imagine if a person tried to hold a conversation with you while maintaining an exaggerated distance between the two of you; you might rightly feel insulted, as if you are not worthy of the other person's standing any closer. Spatial distance means a break in the fellowship, just as his subjects have to bow and stand off some distance when speaking to a king.
But God desires closer fellowship with us than that, and so he takes on human flesh and communes with us on our level. This is because God loves us, and wants us to know him in a manner suited to our capacity for knowing.