Each Set Booster contains 12 Magic cards, 1 Art Card, and 1 token, ad card, Helper card, or special card from Magic's history (a card from "The List"), with a combination of 14 Rares or Mythic Rares, 27 Uncommons, 38 Commons, and 1 Eternal Night Full-Art Basic Land card in every pack.
Being able to get that bonus prize card would speed your win up
Card Text: Ability: Shadowy Concealment If 1 of your Darkness Pokemon is Knocked Out by damage from an attack from your opponent's Pokemon ex, that player takes 1 fewer Prize card
They're just widespread enough that it seems like those who want them will be able to get them, while those who aren't interested won't end up owning too many
Illustrated by Ryota Murayama