Objection 1. It would seem that indulgences do not avail religious. For there is no reason to bring supplies to those who supply others out of their own abundance. Now indulgences are derived from the abundance of works of satisfaction to be found in religious. Therefore it is unreasonable for them to profit by indulgences.
Objection 2. Further, nothing detrimental to religious life should be done in the Church. But, if indulgences were to avail religious, this would be detrimental to regular discipline, because religious would become lax on account of indulgences, and would neglect the penances imposed in chapter. Therefore indulgences do not avail religious.
On the contrary, Good brings harm to no man. But the religious life is a good thing. Therefore it does not take away from religious the profit to be derived from indulgences.
I answer that, Indulgences avail both seculars and religious, provided they have charity and satisfy the conditions for gaining the indulgences: for religious can be helped by indulgences no less than persons living in the world.
Reply to Objection 1. Although religious are in the state of perfection, yet they cannot live without sin: and so if at times they are liable to punishment on account of some sin, they can expiate this debt by means of indulgences. For it is not unreasonable that one who is well off absolutely speaking, should be in want at times and in some respect, and thus need to be supplied with what he lacks. Hence it is written (Galatians 6:2): "Bear ye one another's burdens."
Reply to Objection 2. There is no reason why indulgences should be detrimental to religious observance, because, as to the reward of eternal life, religious merit more by observing their rule than by gaining indulgences; although, as to the remission of punishment, which is a lesser good, they merit less. Nor again do indulgences remit the punishment enjoined in chapter, because the chapter is a judicial rather than a penitential tribunal. hence even those who are not priests hold chapter. Absolution from punishment enjoined or due for sin is given in the tribunal of Penance.
Source: Summa Theologica (New Advent)