It's a straight-forward task, something important when starting a game as complex as Magic
Goryos Vengeance is a mid-tier deck at best, and the optimal version doesnt even run the Yargle and Multani package
But with this, I can just cast it at the end of someone's turn, draw it, and cast it with all of my mana free
Similarly, the power of the creature attacking you is checked only when MacCready's second ability triggers
When the spell or ability resolves, it checks the targets to make sure theyre still legal (that is, theyre still there and they still match the requirements of the spell or ability)
Cards like Doubling Cube also demonstrate how this deviation from usual Magic arithmetic allows for unique and interesting build-arounds its not all just brute force and big numbers getting bigger