Monster class modifiers interact with specific type or group of monsters:
Example1(weapon): +40% damage vs Vampires. Only attacks against vampires get the bonus.
Example2(defensive): -20% to Demon spell damage. Reduce spell damage from group "Demons" by 20%
Example3(utility):+100% more gold from Rats. Rats will drop +100% more gold.(note:Chests and treasure piles are far more realistic than loot drops, however its the genre convention).

Intent: Game zones usually are populated by specific class/group that can accounted by the player and gear adjusted accordingly.
Drawbacks: too much "Monster class specific" gear becomes cumbersome to handle
Typical stats:Restrict powerful modifiers to very specific monster group, with generic groups(such as "Demons") getting average modifiers.