From what I read… FWIF…
Among the reasons why is that they are required to be precept be submitted to the judgement of the priest, and to be given a proper punishment or penance. They have only been forgiven indirectly, they have not been submitted to judgement directly. Though they are absolved as long as all the normal conditions required for absolution were met, they must be submitted to judgment by God’s representative, the priest.
It’s in many ways the same reason one has to confess sins at all, when one might be forgiven already by perfect contrition.
I think the neglect of the ‘penance’ side of the sacrament of penance as well as the ‘judgement’ part that is a particularly common flaw nowadays is what leads to this question sometimes. Every sin is due penance. Every sin must be judged. Penance must be done. A part of it is given by the priest. The rest we have to do too, still.
Submitting mortal sins for judgement is beneficial to the soul because it gains the benefits of grace of a more direct repentance of that sin.
The sin also may be one that requires the priest to exercise some of his other duties, such as lifting excommunication. And often the pastoral advice given for such sins is very helpful. So too it may be that the devil loses some of the power he has over a person when sins are admitted and confessed directly.
‘There remains now the precept of Our Lord to submit all mortal sins to the power of the keys in the Sacrament of Penance; these forgotten sins have not been confessed as yet, nor has the priest pronounced any direct sentence upon them. Though these sins have been remitted indirectly, there still remains the obligation ex jure divino of confessing them directly to the judge in the tribunal of penance when they occur to the mind again, not because these sins have been revived, but because the neglect of God’s command in the matter would involve a new sin. This holds of all mortal sins inculpably omitted, of their species, of all circumstances changing the species, as well as of mortal sins, confessed indeed, but to a priest without jurisdiction who either bona fide or for reasonable motives gave direct absolution of the sins for which he had faculties, thereby remitting the others indirectly.’
‘If the penitent, through forgetfulness or for some lawful reason, without any blame attaching to him, omits to mention something which is necessary for the integrity of the confession, he is bound to disclose it on the next occasion; for, by the decision of the Council of Trent, each and every mortal sin of which one is conscious must be mentioned, that it. may be directly remitted; hence if sins occur to the mind which have not yet been confessed, they must be submitted to the power of the keys. Thus Alexander VII condemned the proposition: Sins which have been forgotten or omitted in confession on account of instant danger to life or for any other reason, need not be mentioned in the next confession (cf. Prop. 11 damn.).’
'In the Sacrament of Penance the remission of sins is effected by the absolution; but sins which have not been mentioned do not directly fall under the absolution since, properly speaking, they are unaffected by the sentence pronounced by a judge who knew nothing about them. Nevertheless the absolution pronounced rite et valide over certain sins is effectual because it is sacramental and because in God’s providence no remission of sin takes place without an influx of sanctifying grace into the soul which presents no obex. Now sanctifying grace removes the whole reatus culpce mortalis and restores a man to perfect friendship with God and to his claim in the heavenly kingdom. Thus valid absolution produces sanctifying grace in the soul and consequently the remission of all mortal sins staining the soul, even those inculpably forgotten. ’
Theory and Practice of the Confessional
