S. What are the effects of this Sacrament [extreme unction]?
T. There are three things, of which the first is to remit sins that still remain after the reception of the other Sacraments, such as those which a man cannot remember, or did not know, and since he doesn't know and remember them, he cannot be sorry for them or confess them. The second is to refresh the sick and give them comfort at a time when sickness oppresses them and the demons vex them with temptations. The third is to restore the health of the body if it would be expedient to their eternal salvation. These three effects are implied in the oil used in this Sacrament, because oil comforts, refreshes, and heals.
[St. Robert Bellarmine, Doctrina Christiana: The Timeless Catechism of St. Robert Bellarmine, Grant, tr., Mediatrix Press (2016), p. 182. I have corrected a couple of minor typos.]