Objection 1. It would seem that an indulgence ought not to be granted for temporal help. Because the remission of sins is something spiritual. Now to exchange a spiritual for a temporal thing is simony. Therefore this ought not to be done.
Objection 2. Further, spiritual assistance is more necessary than temporal. But indulgences do not appear to be granted for spiritual assistance. Much less therefore ought they to be granted for temporal help.
On the contrary, stands the common custom of the Church in granting indulgences for pilgrimages and almsgiving.
I answer that, Temporal things are subordinate to spiritual matters, since we must make use of temporal things on account of spiritual things. Consequently an indulgence must not be granted for the sake of temporal matters as such, but in so far as they are subordinate to spiritual things: such as the quelling of the Church's enemies, who disturb her peace; or such as the building of a church, of a bridge, and other forms of almsgiving. It is therefore evident that there is no simony in these transactions, since a spiritual thing is exchanged, not for a temporal but for a spiritual commodity.
Hence the Reply to the First Objection is clear.
Reply to Objection 2. Indulgences can be, and sometimes are, granted even for purely spiritual matters. Thus Pope Innocent IV granted an indulgence of ten days to all who prayed for the king of France; and in like manner sometimes the same indulgence is granted to those who preach a crusade as to those who take part in it.
Source: Summa Theologica (New Advent)