The Pokemon TCG routinely releases sets following certain themes, including fantastic throwback designs among the most valuable XY Evolution cards, and some valuable Pokemon Tag Team cards in sets like Unified Minds
Players sometimes justify its inclusion through logic such as "If you mill six cards from your own deck, and the only flashback card that you turn over is a Think Twice, you're still going to be even on cards." But in green-blue-black mill-yourself decks that are actively looking to get as much of their library as possible (and in some cases all of it) into their graveyard, finding ways to continue to mill yourself is absolutely crucial
As this indicates, the card was only given out to event staff, making it incredibly rare and sought-after by collectors and explains how it managed to sell for an incredible $16,500 In 2023 on Goldin
This, combined with enough power to keep the card on the MTG banlist for decades, means it's always been one of the most expensive MTG card out there