It is important to note that an indulgence is not redemption and those suffering in purgatory do not need to be redeemed. They are redeemed by the Blood of Christ on the Cross.
That said, it is also important to note that forgiveness, biblically speaking, does not remove punishment. Simply look at David and Moses, all of Israel, the Apostles, all the Martyrs, each and everyone of us. Suffering is part of redemption as the Lord says in the Holy Scriptures. See Deuteronomy 8:5, Proverbs 3:12, Hebrews 12:6, Revelation 3:19, Psalm 94:12, Proverbs 13:24, and more.
Understanding Indulgences can be difficult, especially if you start with the wayward teachings of Martin Luther or the distortions of the separated brethren in general. It is also difficult to understand indulgences if you do not have a full grasp of what it is that humanity fell from and how.
I would need a book to explain it completely so I will try to do it briefly as I can so it fits this format.
What we fell from: The Image and Likeness of God. After the Fall we were still in his image but not his likeness, the likeness is the nature that we fell from where the image would be the Person, which we maintain. Think of the “Communicatio idiomatum” which explains the union of Christ to his humanity. We are in the image of God, and had the nature (likeness) of God, that is a share in his divine life and sonship. This is what we fell from.
How we fell: The sin that was committed in the Garden is the same temptation that each and every one of us face daily and can be described with 3 terms: Sin of the Flesh, Sin of the Eyes and, Pride of Life. There is no sin against God that does not incorporate at least one of these elements. It is the only trick the devil has up his sleeve and it is an effective method of soul stealing.
The above is what we fell from. Now let us look at how we are to return to God and where indulgences connect to the restoration.
- The Grace of God: Grace is a free gift. God's first Free Gift was Creation itself. It was not earned, not merited; even faith was absent from this gift when it was granted. God's un-merited Grace continues throughout the History of Salvation.
There is a cycle that continues that we learned from out first parents when death was introduced into the world. That is to reject the free Grace of God, via the three-fold transgression of Eve mentioned in #2 above, this condition of the human nature is referred to in the Catholic Church as “Concupiscence”. Concupiscence is not a sin in and of itself, but an inclination to succumb to the three-fold temptation.
Through the Grace of God, by the once and for all time Sacrifice of Christ on the Cross, the ministry of redemption through Christ Jesus offers to us the forgiveness of Sins. This forgiveness given to us through Baptism (Water & Spirit) is complete, it forgives us of both Guilt and temporal damage when we enter into Christ’s family. However, because of the 3-fold temptation we are subject to and often do, continue to sin, no different than the sin of Eve. A sin is the rejection of the free Grace and in the New Covenant, a rejection of the Sacrifice of Christ on the Cross. When we do this there is damage to our souls; forgiveness is offered for the Guilt, but the damage, not to God, but to ourselves in regards to a tendency to commit sins, still exists. That damage is Concupiscence.
This is where I have to simplify even more.
In order to fight the 3-fold temptation, i.e. that concupiscence, we as Christians are to do those things that are in opposition to the actions of Eve. These can be explained in this life in 3-fold Reconciliation.
You mentioned in your question CCC 1478, there is in there a theme that you will see in Catholicism, a theme that never goes away and without concentrating can be easily overlooked. In the same way, Martin Luther neglected to see it. Sin hid from us, or a better term would be, Sin closed our eyes and our ears to what is plainly in view. It would be very difficult to not see the sin of Pride in Martin Luther; just about everything he wrote had an arrogance about it. But I digress.
The 3-fold Reconciliation can best be seen in the Evangelical counsels of the consecrated clergy: Vow of Chastity, Vow of Poverty and Vow of Obedience. If we look in CCC 1478 we see Devotion, Penance and Charity. When we prepare for Lent we see Prayer, Alms-giving, and Fasting. There is a thread that ties all these together: Eve, cf. Gen 3:6:
“The woman saw that the tree was good for food [Lust of the Flesh] and pleasing to the eyes [Lust of the Eyes], and the tree was desirable for gaining wisdom [Pride of Life], so She Took ...”
God’s Answer to “Lust of the Flesh”: Fasting, Chastity, Penance.
God’s Answer to “Lust of the Eyes”: Alms-giving, Vow of Poverty, Charity
God’s Answer to “Pride of Life”: Obedience, Prayer, Devotion.
All of these are part of the Reconciliation, reversing the damage that was caused by our sin after we have already accepted Christ. We are completely forgiven as David was forgiven, but God still punished him by taking away his son. Unlike Eve, we receive forgiveness and with that forgiveness comes the punishment that we need from our Father. That punishment comes in the forms listed above while we are in this life. After we die, our punishments are given to us as an act of Love and Mercy so that we may be worthy to walk in the Divine light, understanding completely that which we fell from and that which we are returning to.
Why doesn't the Church just give out the vast wealth of Grace deposit to everyone? Because it would be an action empty of any meaning, of any teaching, of any Love. Martin Luther did not understand this, Martin Luther who struggled with inner demons wanted what many Christians today proclaim: Forgiveness without temporal Punishment, without Mercy and without Justice. We are called to repentance and to make restoration for our transgressions to address the damage done to our souls, not to God, which Christ handled (see Col 1:24). By the Corporal and Spiritual Works of Mercy we make reparations for our acts; if we fail to do this in our lifetime, we shall do so mysteriously in the next. I, for one, do not want to stand before God as I am, a sinner. I wish to be cleansed and to be free from the sinful desires of this life. If I can't do it on my own here, Christ will do it for me, and my brothers and sisters in Christ who Pray for me and do those things Pleasing to God, will help me by example.
There is a relationship between Reconciliation, Purgatory and Indulgences that would take much more to explain here. It is a pursuit whose outcome is worth the journey. The best tool you could bring with you on the journey is an open mind; if it's closed, like in so many areas of life, there is little hope.
I would like to add to this answer, because it helps to clarify the Churches position on the topic of indulgences. The nature of the Graces given to the Church and how indulgence flows from those Graces and regarding the method of its Reception: it is passive. What do I mean? I mean that concerning the 7 Graces or the 7 Mysteries or the 7 Sacrements, the Graces are not works, that is, we do not work for them, we do not take them as Eve did because of a desire. The Graces of the Church which flow from Christ are to be received and not taken, there is no Taking of Grace. There is Nothing prior to the reception of these free Graces. Not Works, or Faith has no value; you can work all day long and it means nothing without the Cross of Christ. When you have Received the Grace, then the spirit of God works in you and you with it. You can not receive an indulgence if your heart is not contrite and you are not open to the gift. By Works initiated by the Holy Spirit working in us because of the Grace that we receive, we merit the indulgence (Penance) which is a Grace of God, which cleanses us of the personal damage caused by sin, making us worthy to be with God.
I hope this helps, indulgences is a difficult subject especially given the history of its abuses.