Great and honorable acts are a great good. This is relevant because, for a holy entity like God Almighty to grant forgiveness to his persecutors, like Paul, is a great and honorable act.1
A true evil-conquering story is a great good. This is relevant because the ability for God to turn the greatest evil into the greatest good is arguably the greatest evil conquering story.1
Genesis 50:20 -- “As for you, you meant evil against me, but God meant it for good in order to bring about this present result,”
Certain relationships (of forgiveness, of empathy, etc.) are a great good, especially in the context of eternity. 1
This is relevant because Jesus's (God's) forgiveness of such individuals as Paul results in a uniquely special relationship which is otherwise impossible to attain.
Luke 7:47 -- “but he who is forgiven little, loves little” Such a relationship is an especially great good and especially for Paul, since it is perpetually enjoyed throughout eternity. Its value is limitless. The love, adoration, and worship that Paul etc. have for Christ will now be of higher quality for eternity. Other people in heaven will also be worshiping extra hard, knowing how God has forgiven Paul and those like him). As Paul says in Eph 3:8 — “To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ,”