What is the use of the Firefly accessory?
In Final Fantasy XII, there exists an accessory named "Firefly". Its sole purpose is to reduce earned EXP to 0. Under what conditions in the game would this be a good idea/required?
The usages of the Firefly accessory are very, very, very minimal... but it does have its advantages in very specific situations.
From past experience, and other articles like this GameFAQs, the most practical usage is grinding to offset enemies who have a "Level X Spell".
If we look at the Sleep Status for example, it states
Level 2 Sleep: Inflicts Sleep to targets whose levels are a multiple of 2.
If you are entering into a boss fight, which you know who uses Level 2 Sleep, you can grind your characters to an odd level. Once your characters have reached an odd level, you can equip this accessory to the character to assist the grinding of the other characters without the fear of jumping to an even level.
Other, but not as practical, usages are
+2 Strength - There are monsters, primarily hunts and bosses, that do not give any experience when defeated. The Firefly accessory gives additional 2 Strength, which could help a bit.
Low Level Challenge - Self explanatory. There are players who love a challenge and want to beat the game at a very low level