One must always remember that a prayer or good work itself is more precious than the indulgence that may be attached to it; for an indulgence is merely the remission of temporal punishment; whereas, a prayer devoutly said means an increase of grace and merit for eternity. There is all the difference between the temporal and the eternal.
—from Purgatory: Explained by the Lives and Legends of the Saints by Fr. F.X. Schouppe, S.J., Appendix 1: Indulgences