The basis for the doctrine of the restoration is the absolute agreement of good/good, and evil/evil in the resurrection. During our mortal experience, we are not always wholly good (nor, thankfully, wholly evil). What we are in this life is sinners, temporarily separated from God and struggling to be obedient but falling frequently into disobedience. Repentance is important because it allows us to move in the direction of more perfect obedience. And certainly, repentance was critical for Corianton.
Alma tells Corianton that repenting now can “restore” good prior to the final judgment. A single failure does not eternally condemn us. If we repent, if we turn away from error and turn toward Yahweh, we may be restored to good, both in this life and the next.