Just a wild guess
Acting as a precursor for Dark Synchros, it triggers an effect upon summoning, bringing forth a regular Red Dragon Archfiend and wiping out all opponent's Attack Position monsters
Also, considering you can use fusion material from the deck when the opponent controls an Extra Deck monster, you can easily put the opponent in situations where they need Extra Deck monsters to beat you, especially since they probably wont expect you to use Shaddoll Fusion in your deck to begin with, causing that requirement to be met
Learn the Basics of Magic: The Gathering First things first: If you dont know how to play Magic , youll need to learn the basics
Like most Pokmon cards, the value of Jirachi depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Generations)