The first thing to note is that forgiveness of a sin is separate from punishment for the sin. Through sacramental confession we obtain forgiveness, but we aren't let off the hook as far as punishment goes.
That's a quote from Catholic Online's article on indulgences. I've never heard forgiveness spoken about in this way. Even the site's tag wiki for forgiveness specifically includes release from punishment. I know this question is similar, but its answers don't cover a Catholic view. The only well-sourced answer explains a Lutheran stance.
If God's forgiveness does not spare one from punishment, then what is it? Does this mean that the sacrament of confession alone does not spare you from punishment? Where does the Catholic Church get this distinction? Is there a biblical basis? Is it part of sacred tradition? Did a council decide it?
On another note, I learned while researching for this question that anyone who has completed the three prerequisites for an indulgence may gain one by participating on Christianity.SE:
- Christian Doctrine. Partial indulgence to those who take part in teaching or learning Christian doctrine.