Since justification in the Catholic Church comes by faith and works through grace (which is in contrast with Romans 11 which tells us that there can be no works in grace), part of that justification process is in attending Mass and essentially gaining spiritual points so that they can spend as little in Purgatory as possible. This is why there is a Mass service every morning so that the Catholics have the opportunity to atone for their sins.
However, outside of this system, there can be no atonement, for in Catholic theology, confessing sins to a Catholic Priest is necessary for justification, and attending Mass is necessary for justification. But to get out of Purgatory in a timely manner, one has to attend a multitude of Masses so that Jesus Christ can be sacrificed for their sins a multitude of times.
To help understand this system, think of the sacrifice of Jesus Christ as being equivalent to that of a goat or a bull, because each time the beasts are sacrificed, they are sufficient to cover the sins that the person has committed since the last sacrifice, and so the same with Jesus Christ in the Catholic system. Every time they go to Mass, the Priest submits their sins to be covered by the unbloody sacrifice of Jesus Christ. But that does nothing to cover the sins of the future, so they will have to come back so that their future sins can be atoned for.
Once again, what I have written here enjoys the support of the Council Of Trent. (Sixth Session, 1547) Canon 30. If anyone says that after the grace of justification has been received the guilt is so remitted and the debt of eternal punishment so blotted out for any repentant sinner, that no debt of temporal punishment remains to be paid, either in this world or in the other, in purgatory, before access can be opened to the kingdom of heaven, anathema.