We all have loved ones who have died, and most of us as Catholics know to pray for their souls. Here is what St. Alphonsus Ligouri says in my little book, Purgatorian Manual:
"The practice of recommending to God the souls in Purgatory, that He may mitigate the great pains they suffer, and that He may soon bring them to His glory, is most pleasing to the Lord and most profitable to us. For these blessed souls are His eternal spouses, and most grateful are they to those who obtain their deliverance from prison, or even a mitigation of their torments. When, therefore, they arrive in Heaven, they will be sure to remember all who have prayed for them. It is a pious belief that God manifests to them our prayers in their behalf, that they may also pray for us. It is true that these blessed souls are not in a state to pray for themselves, because they are, so to speak, criminals atoning for their faults. However, because they are dear to God, they can pray for us, and obtain for us the divine graces. St. Catherine of Bologna, when she wished to obtain any grace, had recourse to the souls in Purgatory, and her prayers were heard immediately. She declared that, by praying to these Holy Souls, she obtained many favours which she had sought through the intersession of the saints without obtaining them. The graces which devout persons are said to have received through these holy souls are innumerable."
He goes on to say that it is our Christian duty to relieve them by our suffrages, for they cannot help themselves and so are in need of our assistance.