Ping!
After the deaths of seven family members and numerous friends, I always pray for them.
A thief can steal money, and even though he had to go to jail, may be forgiven, but the consequences of his action still exist. He needs to return what he stole. He needs to make up for the suffering he caused the owner and even the owner's family.
Same with purgatory. Through the Mercy of Jesus, and the grace of the sacrament, a true penitent receives forgiveness from God. But what we do has consequences beyond our actions. Sin has greater consequences, a ripple effect like a stone thrown in a pond. Until reparation for this is made, a soul facing its God, in all His Perfection would rather some for those consequences, than to face God in that state.
What an unspeakable joy there must be when a soul is released, purified, and seeing its God, never to be separated again!