Imagine your opponent sneaking in Striking Sliver or Bonescythe Sliver with ninjutsu and completely changing the landscape of combat
You can go the value route with something like Combustible Gearhulk, the combat route with Combat Celebrant and Fear of Missing Out, or the get everyone dead route with a damage-based ETB creature like Overlord of the Boilerbilges or Inferno Titan
These two things alone are quite useful little tricks
Sorry if this has been asked before Dyl November 4, 2022, 3:41am 2 Bulbapedia has all of the information that youre searching for and more
The ability allowing it to be cast out of exile is only added when the instant or sorcery adventure resolves if the creature is exiled, or the spell is countered-and-exiled through something like Faerie Trickery, it's not coming back
It's important to remember that, even though each Companion card states you can cast it from outside the game, this is not /actually/ the case