[mensetmanusman]

From your link:

“ Least of all is an indulgence the purchase of a pardon which secures the buyer's salvation or releases the soul of another from Purgatory. “

One might associate it with helping move an individual from a smaller circle to a larger circle of ethics: https://en.m.wikipedia.org/wiki/The_Expanding_Circle

“ Singer discusses the relationship between biological capacity for altruism and morality. He argues that altruism, when directed to one's small circle of family, tribe or even nation, is not moral, but it becomes so when applied to wider circles. “

E.g. charity where you sacrifice your individual goods for the larger community is good for the soul.