danman916 is on the right track as far as the background to indulgences is concerned.

Forget, for a moment, perceptions around indulgences and purgatory, and think instead about the notion of sin and the need for purgation.

As sinners, all people are in need of purgation - that is, the laying aside of the sin that weighs us down. As such, purgatory (while it may be defined in dogmatic terms somewhere), can really just be considered an extension of this life: in other words, a place on the other side of the veil of death where the process of setting sin aside continues until we are ready for the beatific vision.

As indulgences are merely replacements (out of pastoral necessity) for appropriate penances commensurate with our sins, then they are also merely part of that purgative process.

And in this respect, they can be thought of in a way that is actually quite consistent with a Patristic understanding of sin, penance, and preparation for heaven.

Satisfaction

1459 Many sins wrong our neighbor. One must do what is possible in order to repair the harm (e.g., return stolen goods, restore the reputation of someone slandered, pay compensation for injuries). Simple justice requires as much. But sin also injures and weakens the sinner himself, as well as his relationships with God and neighbor. Absolution takes away sin, but it does not remedy all the disorders sin has caused.62 Raised up from sin, the sinner must still recover his full spiritual health by doing something more to make amends for the sin: he must "make satisfaction for" or "expiate" his sins. This satisfaction is also called "penance."

1460 The penance the confessor imposes must take into account the penitent's personal situation and must seek his spiritual good. It must correspond as far as possible with the gravity and nature of the sins committed. It can consist of prayer, an offering, works of mercy, service of neighbor, voluntary self-denial, sacrifices, and above all the patient acceptance of the cross we must bear. Such penances help configure us to Christ, who alone expiated our sins once for all. They allow us to become co-heirs with the risen Christ, "provided we suffer with him."63

The satisfaction that we make for our sins, however, is not so much ours as though it were not done through Jesus Christ. We who can do nothing ourselves, as if just by ourselves, can do all things with the cooperation of "him who strengthens" us. Thus man has nothing of which to boast, but all our boasting is in Christ . . . in whom we make satisfaction by bringing forth "fruits that befit repentance." These fruits have their efficacy from him, by him they are offered to the Father, and through him they are accepted by the Father.64

The punishments of sin

1472 To understand this doctrine and practice of the Church, it is necessary to understand that sin has a double consequence. Grave sin deprives us of communion with God and therefore makes us incapable of eternal life, the privation of which is called the "eternal punishment" of sin. On the other hand every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory. This purification frees one from what is called the "temporal punishment" of sin. These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin. A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain.

Another thing which mildly irks me about indulgences is that they're vaporous and elusive. No matter how perfectly you strive to fulfill the conditions set out, you can never know for sure if you gained the indulgence.

As a child I would read in prayerbooks that if you said thus-and-such prayer you could gain an indulgence for having done so. I thought you had to go to the presbytery and tell the priest you had done so and he'd somehow tell you if you gained it or not.But I didn't know anybody who did that; and that priests would have had no way of knowing, either.

Live, experience, and learn.