1. God allows evil to the great extent for the complete and ultimate destruction of evil and sin through Jesus Christ (Col 2:13–15; Heb 2:14–18; 1 John 3:8).
2. God allows evil so that people who sin can continue living in mercy to repent, because God loves all and wants everyone to repent and be saved (1 Tim 2:4; 2 Pet 3:9).
3. God gives free will and the ability for humanity to choose to flee sin and pursue holiness (Rom 6:15–19).
4. God allows evil and that allows the greatest acts of good — repentance, mercy, compassion, forgiveness, comfort, and relief — that cannot exist otherwise (Luke 5:32; 6:20–49).
5. God allows evil with great suffering for those who suffer greatly can experience the grace of God that gives peace in Jesus Christ (Rom 8:16; Phil 1:27–30; Jas 1:2–4; cf. 1 Pet 3:14; 4:1, 12; 5:10).
6. God allows evil and suffering as God’s grace strengthens a person’s character through suffering that cannot exist without suffering (Rom 5:3–5; 8:16, 28; 2 Cor 12:7–10).
7. God allows suffering to resolve suffering justly in an eternal afterlife by restoring creation from corruption and decay in the redemption of the faithful bodily resurrecting from the dead (Rom 8:18–25; 2 Cor 4:17).
[If you know of another reason from Scripture, please comment below so that I can consider adding it to the list. Thank you.]