Put simply, its an incredible Plan B
If you already have good blink targets, like various Thassa, Deep-Dwelling decks in Standard, then Yorion, Sky Nomad is an incredibly powerful card to have access to in every game
Its one of the most popular long-term paper competitive formats because it rewards mastery
All 19 of them are a master-class in flavor, but I think Heavent Sent and Fugitive of the Judoon are the best examples of this
In each 5-card pack, 4 slots pull from that main card pool, in a variety of rarities, from the normal Super, Ultra, and Secret Rares, to Starlight, Platinum Secret, and the unique "Prismatic"-style Ultimate & Collector's Rares found only in Rarity Collection sets