For one thing, that is what the Catholic Church says about it, and she is the institutiton that gave us Purgatory. I find it ironic that we Protestants are always having New Advent or Catholic Answers or some other website thrown at us whenever there's a debate. The attitude is always "There. That's the real story."...until the Catholics themselves find the official teaching of their church not to their liking. One well-known poster has explained that he can't give a personal opinion on various issues because he is obligated to be in step with the Catholic Church--until it comes to Purgatory, that is, where he says he prefers to think it will be "fine."
Well now, I invite you to go to New Advent where you will find that Purgatory is a place of PUNISHMENT for sins previously committed.
And if you choose not to do that, here's an excerpt from New Advent, the Catholic Encyclopedia--
"That temporal punishment is due to
sin, even after the
sin itself has been
pardoned by
God, is clearly the teaching of
Scripture.
God indeed brought
man out of his first disobedience and gave him power to govern all things (
Wisdom 10:2), but still condemned him "to eat his bread in the sweat of his brow" until he returned unto dust.
God forgave the incredulity of
Moses and
Aaron, but in punishment kept them from the "land of promise" (
Numbers 20:12). The
Lord took away the
sin of
David, but the
life of the child was forfeited because
David had made
God's enemies
blaspheme His
Holy Name (
2 Samuel 12:13-14). In the
New Testament as well as in the
Old,
almsgiving and
fasting, and in general penitential acts are the real fruits of repentance (
Matthew 3:8;
Luke 17:3;
3:3).The whole
penitential system of the
Church testifies that the
voluntary assumption of penitential works has always been part of
true repentance and the
Council of Trent (Sess. XIV, can. xi) reminds the
faithful that
God does not always remit the whole punishment due to
sin together with the guilt.
God requires satisfaction, and will punish
sin, and this
doctrine involves as its
necessary consequence a
belief that the
sinner failing to do penance in this
life may be punished in another world, and so not be cast off
eternally from
God."