Christ's work on the Cross is finished and perfect, there's nothing we need to add to it. But the Sacraments, incl. James White claimed that would be a work, and therefore must be false. All it is is the proof of our willingness to change, to get right with God again. One who is not ready to do anything good cannot truly be sorry for his or her sins.
Penance is to restore our spiritual life.
It is good for US and for others. But it has nothing to do with the forgiveness of sins itself. If you intend to do your penance, your sins are forgiven at confession. Well, Mr. White, we Catholics believe and teach that we are saved by grace alone . God gives us grace.