The rarer the card, the higher the value
For character collectors and Unova fans, it is the definitive modern Rosa card a grail-tier piece that pairs gorgeous festival artwork with top-end rarity
In Extended Dredge, Bloodghast was a card that could be milled into the graveyard with dredge, come back from the graveyard for free with landfall, possibly do some attacking (dealing with four Bloodghasts can actually be super annoying because they just keep coming back), and in the late game make an amazing combo with Bridge from Below and Dread Return, generating a ton of Zombies when they are sacrificed and then returning to the battlefield for free
Mega Gengar EX Deck Dark-type decks are slowly rising in the Pokemon TCG Pocket meta, and Mega Gengar EX is definitely one of the best ones
In the modern TCG era, "Master Setting" (collecting every single card in a set) has become a hobby for the ultra-wealthy