Why are indulgences ever needed for oneself?

Like crimes in legal systems, sins have two consequences: they make me guilty (the conviction) of trespassing God's law, and they involve a punishment (the sentence) for such trespassing. In the Sacrament of Penance, my conviction is removed through the forgiveness of Christ, whereas the penance give to me by the priest commutes my punishment. Thus, if one participates in the Sacrament of Penance throughout his life, and is judged worthy to go to Heaven in the Particular Judgment, there should be no need for Purgatory. I understand one can gain Indulgences for others, who perhaps did not have their sins forgiven and sentences paid for during their life on Earth. But if we pay all our penances on Earth, what is the personal use of indulgences? Is it a kind of insurance for, say, one dies unexpectedly, without having confessed some sins?