Unusually because it doesn't force all your other goblins to attack
The resulting spell's color, mana cost, mana value, and so on are determined by only those alternative characteristics
Beyond the draft setting, having access to this ability in the command zone for 6 mana is just kind of bonkers
That ability is "When the last defense counter is removed from this permanent, exile it, then you may cast it transformed without paying its mana cost." As a Siege enters the battlefield, its controller chooses an opponent to be its protector
Master Your Opening Turns: MTG Opening Hand & Probability Simulator How consistent is your decks mana base
It takes very little effort to make this a game-ender, especially if you pair it with cards like Primeval Titan and Reshape the Earth, and it even got labeled as a Commander Game Changer