In order to take Adam’s place
Another requirement of our reconciliation with God was this: that man, who by his disobedience had become lost, should by way of remedy counter it with obedience, satisfy God’s judgment, and pay the penalties for sin. Accordingly, our Lord came forth as true man and took the person and the name of Adam in order to take Adam’s place in obeying the Father, to present our flesh as the price of satisfaction to God’s righteous judgment, and, in the same flesh, to pay the penalty that we had deserved.
In short, since neither as God alone could he feel death, nor as man alone could he overcome it, he coupled human nature with divine that to atone for sin he might submit the weakness of the one to death; and that, wrestling with death by the power of the other nature, he might win victory for us.
The Institutes of the Christian ReligionBook II, Chapter XII, Number 3