Mostly these are big creatures that get discounts for each creature in your graveyard
Each one has a different color identity and ability, but sometimes it's fun to build in a single color
Ugin's Labyrinth helps smooth out early draws by providing a quick mana boost for powerful early game interaction like Chalice of the Void, which can shut out some strategies altogether
(Whenever a creature you control attacks alone, it gets +1/+1 until end of turn for each instance of exalted among permanents you control.) A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step (including creatures controlled by your teammates, if applicable)
Fell Specter isnt strictly worse than Megrim or Liliana's Caress, its just that it costs more and its more vulnerable to creature removal and/or wraths
The most valuable among those bonus cards is this reskinned Snapcaster Mage, which could be found in roughly 1 out of every 100 copies of the Secret Lair x Hatsune Miku: Electric Entourage drop