Grixis Affinity floods the board with cheap artifact creatures, and Refurbished Familiar (a 2/1) and Krark-Clan Shaman (a 1/1) are the low-toughness part of that swarm. Breath Weapon's 2 damage to each non-Dragon creature sweeps those away. It does not kill the bigger bodies like Myr Enforcer (4/4) or Utrom Monitor (3/3), so it is a way to clear the small creatures rather than the whole board, and it is best cast before Galvanic Blast gets metalcraft. Call Damage Control lets us get back creatures lost to Galvanic Blast and Cast Down, or cards discarded to Refurbished Familiar, keeping Tron assembly on schedule. Barrels of Blasting Jelly and Crop Rotation are too slow against a deck that wins by going wide and fast, so sweeping the small creatures and rebuilding is the priority.