God Mode Sue exists purely to show up how pathetically weak the rest of the world is, and how badly they need his or her help. If there's anybody else that is even capable of standing up for themselves, they may lose their abilities for some reason when the character comes into the equation, or become completely incompetent, or both. They'll probably get captured or find something that they just can't handle. Then the God Mode Sue shows up, saves the day on his or her own at least twice as easily as they usually do when working as a team, and doesn't get his or her ass kicked at all. Then he or she stands around and wallows in their praise a bit.
God Mode Sue is so powerful because he or she often gets New Powers as the Plot Demands, and without any explanation or any of the normal limitations that a good writer will insert. In most balanced fiction, we generally know the hero's going to win, Downer Ending aside (Status Quo Is God, after all), but their abilities may fail them (or something like that) and so we watch to find out HOW they do it. God Mode Sue's powers never fail, nor is there any point where you think, "I KNOW the hero's gotta win, but how are they going to get out of this mess?" The plot and the fight is so blatantly biased towards the God Mode Sue that it becomes rather boring (and sometimes insulting).
It's worthy to note that there is a correlation between badass and this trope. A proper badass will frequently break the Willing Suspension of Disbelief, but they generally sell it through either Refuge in Audacity or just generally being convincing as something that could naturally come up with the character. However, they are still treated as normal characters by the plot and have to face potential repercussions to their actions like anybody else. God Mode Sue... doesn't
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