This is a crucial element of the doctrine of restoration. The basis for the doctrine is the absolute agreement of life:resurrection, good/good, and evil/evil. While those absolutes are understandable, they do not really tell us about our experience on earth, for we are not always wholly good (nor, thankfully, wholly evil). What we are in this life is sinners, and we are temporarily apart from the way of the Lord. If the doctrine of restoration is only in the absolute, we sinners would be condemned to be restored to evil rather than good. This is where repentance is important. In particular, repentance is critically the issue for Corianton, as he needs to avail himself of the opportunity to repent.
Alma tells Corianton that repentance in this life can “restore” good prior to the final judgment. A single failure does not eternally condemn us. If we repent, if we turn from the way of error and return to the way of the Lord, we may yet receive the reward of the Good. In this life, we may be restored to Good.