Are we bringing this back up again? I'll answer it anyway:
Jesus asks us to be perfect as His heavenly Father is perfect. But we know that He still has quite a bit of work to do in us. The Bible tells us that we sin daily, and since nothing unclean will enter heaven (Revelation 21:27), we know that there can be no sin there. So Jesus must make us perfect. If we haven't allowed him to make us perfect with our time on earth, then He will finish the work of our sanctification before we enter heaven. This is why St. Paul says that some are saved, but only after passing through the fire (1 Cor 3:15) This "fire" is purgatory.
Purgatory is an application of the graces merited for us by Jesus on the Cross, so that we might be made pure before entering heaven. It can be thought of as the final stage of sanctification (i.e. the process of becoming holy). It is the application of the blood of Christ to our souls, making us really pure. God does no simply say we are pure, He makes us so, since His work accomplishes that which He intends (Isaiah 55:11) Purgatory does not add to or take away from the work of Jesus. It is the work of Jesus. It is one's immersion into the love of Christ, which removes the residue of imperfection from us.
The Bible tells us that God is like a "refiner's fire" (Malachi 3:2) A refiner's fire burns all the corrosion and rust off of metals to make them shine with the fullest glory. Purgatory is God's refining or purification of souls that have died in His grace but still have "impurities" within them. It is not a second expiation (or atonement) for sin or a second chance for bad Christians. Rather, it is an application of the one perfect expiation of Jesus on the Cross; it is for those who die in God's grace but have not been made perfect.