I'd been wondering how to reconcile the concept of purification, with how help can come from prayers from others during that purification (given that it is an act necessarily individualistic), and now I find that Cardinal Ratzinger addresses this in his book "Eschatology":
Does not this prayer [for the departed] presuppose that Purgatory entails some kind of external punishment which can, for example, be graciously remitted through vicarious acceptance by others in a form of spiritual barter? And how can a third party enter into that most highly personal process of encounter with Christ, where the "I" is transformed in the flame of his closeness? Is not this an event which so concerns the individual that all replacement or substitution must be ruled out? Is not the pious tradition of 'helping the holy souls' based on treating these souls after the fashion of 'having' - whereas our reflections so far have surely led to the conclusion that the heart of the matter is 'being,' for which there can be no substitute? Yet the being of man is not, in fact, that of a closed monad. It is related to others by love or hate, and, in these ways, has its colonies within them. My own being is present in others as guilt or as grace. We are not just ourselves; or, more correctly, we are ourselves only as being in others, with others and through others. Whether others curse us or bless us, forgive us and turn our guilt into love- this is part of our own destiny. The fact that the saints judge means that encounter with Christ is encounter with his whole body. I come face to face with my own guilt vis-a-vis the suffering members of the body as well as with the forgiving love which the body derives from Christ its Head."The intercession of the saints with the Judge is not...some purely external affair whose success is necessarily doubtful since it depends on the unpredictable benevolence of the Judge. It is above all an inner weight which, placed on the scales, can bring them to sink down.