An indulgence is the remission before God of the temporal punishment due sins already forgiven as far as their guilt is concerned, which the follower of Christ with the proper dispositions and under certain determined conditions acquires through the intervention of the Church which, as minister of the Redemption, authoritatively dispenses and applies the treasury of the satisfaction won by Christ and the saints. (norms, 1)It seems to me that the practice of indulgences depends on a few things. I think of three:
First, it depends on the idea that expiation is something apart from forgiveness and pardon. Sin injures the creation, and though a sin be forgiven and absolved, God's justice demands that the injury be somehow corrected or undone. This is accomplished through acts of penance, the good use of the sufferings of this life, or the purification of purgatory thereafter. An indulgence remits this responsibility to expiate the injury we do to the universe by our sins.
Second, it depends on the very basic assertion of Christianity, which the document also makes, that the Church is "minister of the Redemption of Christ" (38) I think it's easy to have the idea that redemption and salvation is something basically transacted between the individual soul and God ("Jesus my personal savior") and that the Church exists as a more or less human institution to promote and encourage this. A Catholic ecclesiology is much deeper than that, of course. Such would assert that the Church, as the Mystical Body of Christ, is herself the mediation of the salvation God wills for the world and which we have in the person of Jesus Christ. Therefore, it pertains to the Church to minister this redemption. It is in this sense that we can understand extra ecclesiam nulla salus, 'outside the Church there is no salvation.' As the Catechism explains, this phrase "means that all salvation comes from Christ the Head through the Church which is his Body." (846)
Third, the doctrine and practice of indulgences depends on a strong and spiritual sense of the communion of saints. All the baptized are in communion with each other. Or, as Indulgentiarum doctrina puts it, "[t]here reigns among men, by the hidden and benign mystery of the divine will, a supernatural solidarity." The sins of one are an injury for all, but the merits and salvation of each are also a benefit to all. From this communion or solidarity derives a certain fungibility of grace on which the idea of indulgences depends. The Church, as "minister of the Redemption of Christ" can apply the merit of one to another. This communion of saints is catholic in the sense of embracing all of time and space, and so the individual Christian, as a member of the Church, can apply an indulgence gained to one of the faithful departed. (norms, 3)