When you take souls out of purgatory, you get benefits. Not economic, of course.
In the first place, you are doing a very powerful work of charity. There is no greater spiritual benefit for another than to take him out of purgatory.
Secondly, an act of faith: Can you see the souls in purgatory? If the answer is "yes", I am sorry for you. For I am writing this on July 3, 2020, the feast of St. Thomas the Apostle, and our Lord Jesus Christ said to him, "Blessed are those who have not seen and have believed" (Jn. XX, 20).
In the third place, you put your hope in work, since you are certain that we have no permanent city here (Heb. XIII, 14). If we have our hope in Christ only in this life, we are the greatest fools of all men (I Cor. XV, 19).
Fourthly, as an investment it is not a bad one. You have in Heaven interceding for you all the souls for whom you have offered plenary indulgence. You accumulate for Heaven abundant savings, with complete certainty of one day receiving the spiritual benefit, because it is there that neither moth nor rust corrodes what you have deposited (Mt. VI, 19-21).
Fifthly, because in this brotherhood the first ones for whom the prayers of the other members of the brotherhood are offered are for the deceased members of the brotherhood and their direct relatives.
But above all, God rewards and makes you grow in charity.. When you pray for others, the main beneficiary is not the other. IT IS YOU. You will see how, little by little, by offering each day a plenary indulgence for a soul in purgatory, God will make charity grow in you. The reward is bestial: "The King will say to those on his right hand: "Come, blessed of my Father, and inherit the Kingdom prepared for you from the beginning of the world, for I was hungry and you gave me to eat, I was thirsty and you gave me to drink, I was passing through and you gave me lodging, naked and you clothed me, sick and you visited me, in prison and you came to see me". The righteous will answer him: 'Lord, when did we see you hungry, and feed you; thirsty, and give you drink? When did we see you passing through, and lodge you; naked, and clothe you? When did we see you sick or in prison, and come to see you?' And the King will answer them: '"Truly I tell you, whenever you did it to the least of my brethren, you did it to me'." (Mt. XXV, 34-40). These small and defenseless ones are also the souls in purgatory.