Let's imagine your opponent has 1 HP left. You have 2 creatures out. First to attack is your Ash Eater, who does 2 damage. In a normal situation, the game would now end. But if it ends, we don't know how much damage you did for the bonus, so the game finishes the turn instead. Your other creature, however, is an antimattered Massive Dragon. Whoops. Now your opponent didn't die after all.