...and the reward for beating it is a useless medal.
Parodied: The Bonus Boss gives you nothing but Vendor Trash, with the game giving you the justification that there's not much they can give you.
How useful the item is is dependent on how you've set up your party.
...Or how high a level you are. If you're at level 1 or 99, it's a Game Breaker. However, if you're at levels 40-60, it's useless.
The item opens a Bonus Dungeon and the reward for beating it is a useless medal. That medal actually has to be delivered to a father and son named Bugand Error, respectively. The reward for this quest is... a magnifying glass.
Averted: Either the item is relatively useful, or there is no reward.
Enforced: "All players love a really difficult challenge, we should add a boss just to mess with speed runners, completionists, and offer a chance for bragging rights."