Emerald cards often punish opponents by forcing them to discard cards, making them a great counter to aggro decks
I believe the actual reason behind them is the following
Something that also worked in Pokmons favor was that their last two sets, Twilight Masquerade and Stellar Crown, both had been fairly popular sets after the first five sets of the Scarlet & Violet era cards had been priced and sold much lower than MSRP and were usually close to distribution cost for stores at launch
Use it for CCGs, TCGs, living card games, expandable card games, prototype card games, or even educational flashcard projects
Destined Rivals is such a big set full of beloved Pokemon and Trainers, that it's natural for certain superstars like Mewtwo to steal the show