From the book Purgatory Explained by the Lives and Legends of the Saints by Fr. F.X. Schouppe, S.J. :
"On this subject the Church proposes two truths clearly defined as dogmas of faith: first, that there is a Purgatory; second, that the souls which are in Purgatory may be assisted by the suffrages of the faithful, especially by the Holy Sacrifice of the Mass."
I was thinking that most Christians would agree that you can't audit the class Life 101. There is a grade of some sort given at the end. The existance of Purgatory adds the statement, it ain't pass/fail either.