It takes a little while to transform Sidequest: Hunt the Mark into Yiazmat, Ultimate Mark, but you do get a removal spell up front, and it costs nothing to get your free Treasures
With your current hand, you can do everything from adding Pokmon to your bench to retreating your active Pokmon to your bench to attaching an Energy card to a Pokmon to playing your Trainer cards
Summoning sickness is a huge downside, especially since Vannifar has no means of protecting itself, but haste or a bit of luck turns this into a value or combo engine, depending on how you built your deck
That Crdan keeps doing it is what really drives up his stock
On top of this, every Radiant Pokmon is rotating as well, which means that players won't be able to use Radiant Greninja for draw power or Radiant Charizard and Radiant Alakazam for utility damage