It has an instant kill mechanic and makes your assassin cards cheaper to play
Keldeo serves as a secondary attacker, useful against stallers like Oricorio that Mega Gyarados EX cant handle

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Qliphort Genius X EARTH Link 2 [ Machine / LinkEffect ] ATK 1800 DEF - 2 Machine monsters This Link Summoned card is unaffected by Spell/Trap effects and the activated effects of other Link Monsters
The Actual Story: Purchase Limits What people are running into at Costco is not absence
its cheaper but has the legendary sorcery restriction, though thats really not a hard hurdle to pass