While we know (and the Holy Souls in Purgatory know) that the pains of Purgatory will end and all who are in Purgatory will enter into Heaven, we are still bound by charity to try to lessen the suffering of the Holy Souls through our prayers and deeds. While our first responsibility, of course, is to those people we have known, not everyone who ends up in Purgatory has someone to pray for him. Therefore, it is important to remember in our prayers those souls who are most forsaken.
Prayer for Mercy on the Souls in Purgatory
My Jesus, by the sorrows Thou didst suffer in Thine agony in the Garden, in Thy scourging and crowning with thorns, in the way to Calvary, in Thy crucifixion and death, have mercy on the souls in purgatory, and especially on those that are most forsaken; do Thou deliver them from the dire torments they endure; call them and admit them to Thy most sweet embrace in paradise.