The artwork is beautiful, but the card's features are what really increases its value
If you cast both sides, you're casting them together as a single spell all at once, not as two separate spells at two different times
It's a solid inclusion in Dinosaur Tribal decks like Gishath, Sun's Avatar, it's an undercosted 7/7 so it's great in Ghalta, Primal Hunger, and it has a whopping 12 mana cost so it's great in CMC Matters decks that run stuff like Nissa's Revelation
This series would take other properties and bring them to life as Magic cards
(Auction Page) Although Charizard gets most of the attention (and lets face it, he deserves it), his water-type contemporary Blastoise is still one of the most popular and recognizable Pokmon ever, and this is reflected in his value as a collectible