Here are some excellent reasons to gain indulgences (and to offer them for the Holy Souls in Purgatory):
"Father Faber, in his beautiful book “All for Jesus,” enumerates six advantages which accrue to us, from our giving over our Indulgences to the holy souls in Purgatory.
First, it considerably increases our merit, and consequently our claim to glory.
Next, it lays the soul that we release under a particular obligation to us, both because of the singular benefit it receives from entering all the sooner into glory, and also because of the tremendous sufferings from which it is delivered.
Moreover, it gives us the consolation to think that those whom we have released from Purgatory, are doing for us in heaven the great work of loving, praising, and glorifying God on our behalf.
Again, it adds fresh joy to the Church triumphant, from the fact that to the heavenly hierarchy a new citizen is added who can sin no more, whilst to the Church militant it brings comfort from the gain she has made of a new advocate.
Besides, it secures a prompt application of our Indulgences, which, in the possible case that we were in no want of them for ourselves, might remain for many years buried in the treasury of the Church.
And last of all, it entitles us to a speedy discharge of our own debt in Purgatory; for, if temporal alms are satisfactory above most other good works, much more will spiritual alms be so. And if he who gives up anything for God receives a hundredfold, we may have a security that, to recompense us for our generosity. He will so deal with us, that we shall need
little Purgatory, or He will inspire devout souls to pray for us."
- ‘Indulgences: Their Origin, Nature and Development’