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Simply put, madness lets you play a card for an alternate (often cheaper) cost if its being discarded

Then you can sac it for six mana but not tap it to Tutor and drop an Equipment to the battlefield, like half of a Tooth and Nail - great in the very popular Equipment brews too

Combine this with an inflated interested in collectible TCGs' rarest targets spurred by the pandemic and, while nowhere near its peak, maintaining a fair bit of popularity on streams and YouTube

There are also a lot more answers that you could add

Lorcana (lore + arcana) is a magical realm where players take on the role of Illumineers and use magical ink to summon "Glimmers" (copies) of Disney characters from various stories, some familiar and some with bizarre twists, to help them collect lore (victory points)

Just like changelings, these cards automatically fill whatever role you need them to fill, but only one