Every 5 Trophies unlocked moves you up one level and give you a different Mega Reel depending on the level you've just unlocked!
Levels 1-5: You can win 5+ Free Spins.
Levels 6-10: You can win 10+ Free Spins.
Levels 11-15: You can win 15+ Free Spins.
Levels 16+: You can win 20+ Free Spins.