According to the traditional view, souls in purgatory are in a process of understanding their own sins and can only progress in that through the prayers of the living. HOWEVER, remember they exist in a place between time and eternity, so if you pray for them, they get into Heaven (seconds in time could feel like eternity to them!) and they can then pray for you once they are in Heaven. I like to think of it as a stop-gap processing center to correct and perfect. It is probably more efficient to do that on Earth before we die (the intended purpose of the confessional) but not everyone has that luxury. If they weren’t raised Catholic, they may have more processing to do.
Just remember we are talking about people who exist outside of time. One sure thing: If they get to Purgatory, they will eventually make it to Heaven…guaranteed. Since we don’t know when that will be (could be tomorrow; could be yesterday) I cover my bases with an agreement that I will offer prayer for them if they would please pray for me upon arrival in Heaven.
Hey, it’s metaphysical. It might not make much difference, but it can’t hurt!
Pax!
Maria