I think, and I am no apologist, that our Lord made a sacrifice on our behalf to satisfy divine justice.
After this sacrifice to satisfy divine justice, we were given the right to have our sins forgiven through confession. Even though, God in his mercy now forgives our sins, if we fail to satisfy divine justice through works, prayer, and sacrifices, we satisfy that debt to the last farthing in purgatory.
Our works, and sacrifices, are far more efficacious while still in the church militant, than when in purgatory. We cannot pray for ourselves while in purgatory, which is why the church militant is asked to do so. The answer to your question though, as I understand it, lies in satisfying divine justice.