Non-Catholic Christians often misunderstand purgatory as a second chance at heaven for damned souls. Nothing could be more untrue! Damned souls go someplace warmer. Purgatory is only for folks who have ‘died in friendship with God,’ which is a Catholic phrase that means ‘born again.’
Purgatory is like a mud-room. The mud-room is at the entryway to a midwestern home. It’s the place where you take off muddy boots and manure covered barn jackets, and put on something cleaner. Using the mud-room makes you ready to walk through the home’s kitchen and living room without getting yelled at for tracking in mud. Purgatory is like that, since it is the place where a soul can get ready for the bliss and holiness of heaven.
Many souls are just not ready to meet God, but they are trusting souls who have tried to follow God in the best way they knew how. They may not have known much, like the good thief on the cross. Or they may have been too proud or arrogant or simply lacked insight, so they may have committed serious sins without being aware of them as something they need to repent of, and confess to God (and the priest) about.
My current devotional reading is a devotional book about the ‘holy souls’ in purgatory. It gives another reason for purgatory— to get souls less attached to worldly things. Imagine an older woman who dies, but is constantly fretting over what her daughter-in-law is doing with her house and possessions. She needs to set her mind on heavenly things and not the horrible wallpaper her daughter-in-law chose for the front bedroom!
Some Christian souls, like martyrs, are deemed to be ready for heaven straight off. Jesus said to the good thief that he would be in Paradise that day. So, either Jesus considered purgatory a part of heaven (the mud-room?) or else the thief was given the grace to go direct to heaven or perhaps spend only 20 seconds in purgatory to get ‘ready.’
C. S. Lewis is considered by many an authoritative model of the modern Protestant Christian, but he admits to a belief that ‘something like’ purgatory is needed to make us fit for heaven.
There are Bible verses held to speak of purgatory. An article by apologist Dave Armstrong lists some of these verses. I would suggest that you read the article to understand more about the Bible and purgatory.
25 Descriptive and Clear Bible Passages about Purgatory: https://www.ncregister.com/blog/darmstrong/25-descriptive-and-clear-bible-passages-about-purgatory
The important thing about purgatory is that it is not a substitute for accepting Jesus Christ as your savior now, or living a Christian life now, or avoiding sin now. Purgatory is for the ‘holy souls,’ not for people who want to ‘have fun’ now and worry about their souls later. When ‘later’ comes, in the form of death, there is no more mercy available for the damned soul. No damned souls are in purgatory, any more than they are in heaven.
As a Catholic convert who was not brought up on belief in purgatory, and who once knew a lot of (often silly) arguments against it, I find myself a little behind on knowing the concept. I recommend two devotional books by Susan Tassone and published by Our Sunday Visitor, Inc, for other Catholic converts wishing to gain greater knowledge of purgatory and the Holy Souls. [Where do the ‘holy souls’ get their ‘holy?’ Jesus, of course!]
Thirty-Day Devotions for the Holy Souls – https://www.goodreads.com/book/show/809593.Thirty_Day_Devotions_for_the_Holy_Souls
Day by Day for the Holy Souls in Purgatory – https://www.goodreads.com/book/show/23490846-day-by-day-for-the-holy-souls-in-purgatory