I was asked the same question by players who are new to Fate: “What mechanical effect do aspects have besides earning or spending Fate Point?” In Fate Core and FAE, there is a good answer: The aspects are true and established fact in the game. Then some players asked me next question. “Then why do you compel aspects(especially situation aspects) if you get already their effect?”
Here is my answer : Aspects are true and fact, obviously. You can burn yourself if you rush into the “On fire!” zone naturally, or you can’t use your weapon while you are “Disarmed” without spending fate point or rolling dice.
Then when do aspects need to be compelled? When the new stories evolve from the aspects. If you compel “On fire!” aspect, there will be not only a fire: there will be a child who is trapped in a room on fire, or the exit will be blocked by fire. If you compel “Disarmed” aspect, not only you can’t use the weapon, but also have a problem with it : Your weapon will be thrown off a cliff, or snatched by an enemy.
So, aspects(especially situation aspect) are equivalence of other RPG’s condition, or tag, modifier, etc. but it will make a new story when you compel them.
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