According to St. John Vianney:
“If one knew what we may obtain from God by the intercession of the Poor Souls, they would not be so much abandoned. Let us pray a great deal for them, they will pray for us.”
The Souls in Purgatory cannot pray for themselves, but they can still pray for others who request their prayers.
@Cecilia, according to St. Catherine of Genoa, the greatest pain of purgatory is separation from God. But this separation is finite. Remember that the Soul has seen God in person for their particular judgment, and desires to be with God forever… but they cannot yet enter heaven.
"Since the souls in purgatory are freed from the guilt of sin, there is no barrier between them and God save only the pains they suffer, which delay the satisfaction of their desire. And when they see how serious is even the slightest hindrance, which the necessity of justice causes to check them, a vehement flame kindles within them, which is like that of hell. They feel no guilt however, and it is guilt which is the cause of the malignant will of the condemned in hell, to whom God does not communicate his goodness, and thus they remain in despair and with a will forever opposed to the good will of God.