With all of God’s knowledge and power, why did he need Jesus to atone and sacrifice for our sins?
In teaching the people of Gideon, Alma the Younger responded to very much the same question thus:
“Now the Spirit knoweth all things; nevertheless the Son of God suffereth according to the flesh that he might take upon him the sins of his people, that he might blot out their transgressions according to the power of his deliverance; and now behold, this is the testimony which is in me.” (Alma 7:13).
In the forty-second chapter of Alma, Alma the Younger explains to his son Corianton why this expiatory process is necessary and how it functions. In short: people need a higher power than themselves to redeem them from their own sinful natures and the full effects of their sins; however, to satisfy justice, the natural consequences of our sins need to be paid by somebody. Christ offered Himself to be that “somebody”, and in so doing, offers us the higher power we need to be redeemed and sanctified. I would highly recommend that you read that chapter (and, in fact, Alma’s entire sermon to Corianton consisting of chapters 39-42), as I really can’t explain things better than Alma himself does.