If you pick path D, this toy grants you the total of extra +30 Blue Magic Skill. It can be situationally useful to offhand(or mainland) Iris when you're off to Blue Spells farming. The extra +30(or +60 if you dual wield 2 of them) Blue Magic Skill will slightly boost your chances of learning spells, and we all already know that farming for Blue Spells requires all the luck/support we can get.