The problem is, we keep making new enemies all the time. Without some mechanism whereby some of our enemies can stop being enemies society would be impossible, as we would all nurse every grudge to the grave and keep adding new ones. Without forgiveness, for example, the UK would still be at war with virtually every country in Europe and much of the rest of the world. On a personal level, I would have long since disowned all of my family over petty disagreements. Sooner or later you have to simply accept that holding a grudge benefits no-one, it is simply amplifying the repurcussions of the original transgression.

As regarding the criminal justice system, forgiveness is irrelevant. The purpose of the criminal justice system is to prevent the offender from commiting further crimes against society, not to exact vengeance on behalf of the victim, so the question of forgiveness is a personal matter between the victim and offender.