I think the reasoning for corporal mortification is, that we will all go to purgatory to pay for our sins. But while on earth we can pay for our sins here to make the path to heaven easier and faster when we die.
We can't pay for our sins - only One Person in all of time and creation could do that, and has. (all quotes KJV)
Romans 10 (9) "That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved."
Hebrews 10 (27) "And as it is appointed unto men once to die, but after this the judgment: (28) So Christ was once offered to bear the sins of many; and unto them that look for him shall he appear the second time without sin unto salvation."
There is no purgatory - you die, you're judged. Not "easier" or "faster"; but immediately pass or fail, yes or no: accepted Christ or didn't. ALL the sins of mankind have already been paid-for; judgement rests solely upon that one decision.
JPII himself has said that all suffering in the light of Christ is salvific...this is a Catholic perspective not just the view of Opus Dei.