Why did God have to reconcile the world back to Himself through Christ?
God reconciled the world through Christ because humanity was lost in Adam and needed to be brought back to its Creator. Only the God of creation, taking on flesh, could taste death for every man and pay every human being's sin debt. In Christ, God was reconciling the world to Himself, not imputing trespasses.
Les teaches that God was in Christ as He hung on the cross, and that it was the Creator of the universe who hung there. Jesus of Calvary was the same God of Genesis 1:1, who called everything into being. He had to be that God, or He could never have tasted death for every man or paid the sin debt of every human being. Paul is telling us that God was in Christ, reconciling the world back to Himself after He lost it in Adam, not imputing their trespasses unto them. Les also explains that until a lost person dies and loses his opportunity for believing the Gospel, his sins are not being held against him.
To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation.