Evil brings out the best in people. Yes, you read that right. Without true evil to fight, Superman would spend his life getting cats out of trees. If an object did not absorb some of the light that falls on it, and cast no shadow, it would be invisible.
And this is the philosophy of some villains. Yeah, their job is thankless and unpopular - but they press on yet, casting the shadows by which the path of truth is shaped. More commonly still, it's an empty rationalisation they use when challenged on their behaviour.
See also Balance Between Good and Evil, which is usually more supernatural.
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The Joker, especially in the comics, practically defines himself in his opposition to Batman, and feels they need each other, and their eternal struggle of good vs evil, to exist.
Zoom, the second Reverse-Flash, works on the principle that he's making the heroes greater by giving them tragedy to overcome.
Inverted when the original Professor Zoom returned. Because his powers came from the Flash, the one thing he could never do was the only thing he wanted to do: kill Barry Allen.
So long as there is the Sentry, so too must there be the Void.
"Life, which you so nobly serve, comes from destruction, disorder and chaos. Now take this empty glass. Here it is: peaceful, serene, boring. But if it is destroyed..." (Pushes the glass off the table. It shatter on the floor, and several small machines come out to clean it up) "...Look at all these little things! So busy now! Notice how each one is useful. A lovely ballet ensues, so full of form and color. Now, think about all those people that created them. Technicians, engineers, hundreds of people, who will be able to feed their children tonight, so those children can grow up big and strong and have little teeny children of their own, and so on and so forth. Thus, adding to the great chain of life. You see, father, by causing a little destruction, I am in fact encouraging life. In reality, you and I are in the same business."
Legend. Just before he's apparently destroyed by the light:
Darkness: You think you have won! What is light without dark? What are you without me? I am a part of you all. You can never defeat me. We are brothers... eternal!
Mihail Bulgakov's novel The Master and Margarita has Professor Woland, an avatar of Satan, schooling Matthew Levi: "You spoke your words as though you denied the very existence of the shadows or of evil. Think, now: where would your good be if there were no evil and what would the world look like without shadow? Shadows are thrown by people and things. There's the shadow of my sword, for instance. But shadows are also cast by trees and living things. Do you want to strip the whole globe by removing every tree and every creature to satisfy your fantasy of a bare world? You're stupid."
The Stainless Steel Rat's Revenge. After being criticized for going on a crime spree, "Slippery Jim" diGriz explains that the government will reimburse the institutions he robbed, and that the crimes provided excitement for the populace, increased the sale of newspapers, provided exercise for the police and the opportunity for field exercises by the military. He suggests that he should be paid for this instead of punished.
When Harry gets possessed, he gives a plausible argument to the Fallen Angel (Lasciel's imprint) that in possessing him, there's a risk of making him a better person if he survives with his sanity. Given the circumstances, it's not implausible. Later not only is he a greater force of good, but he ends up turning her into a force of Good once more as she willingly dies to save Harry.
The major theme of Good Omens is that the forces of Heaven and Hell should be balanced rather than letting one win over the other.
Lord Vetinari explains it to Captain Vimes at the end of Guards! Guards! this way:
'I'm sorry if this offends you,' he added, patting the captain's shoulder, 'but you fellows really need us.'
'Yes, sir?' said Vimes quietly.
'Oh, yes. We're the only ones who know how to make things work. You see, the only thing the good people are good at is overthrowing the bad people. And you're good at that, I'll grant you. But the trouble is that it's the only thing you're good at. One day it's the ringing of the bells and the casting down of the evil tyrant, and the next it's everyone sitting around complaining that ever since the tyrant was overthrown no-one's been taking out the trash.'
Religion and Mythology
Defied by the apostle Paul in The Bible, against people who rationalized this as an excuse to keep doing things they know are sinful:
Romans 6:1-2: What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it?
A different spin on this is pretty standard Christian theology, though: simply put, you can't truly do good if you don't have a choice to do evil. Hence all the bad things in the world—if God got rid of them, he would be taking away our free will so we would be just robots.
Disgaea 3. The entire plot of the game was planned out by the Super Hero Aurum, the greatest hero of mankind, in order to raise Mao into being the Strongest Overlord... because, having defeated the PREVIOUS Strongest Overlord, Mao's father, he found himself bereft of a purpose. At the very end, he gives a whole speech about how the one thing a hero truly NEEDS, is a VILLAIN. Without an evil to fight, a hero is just a man - unimportant and soon forgotten. Because of that, Aurum spent 200 years disguised as a demon, raising Mao to be evil and powerful, and pushing him towards genocidal anger against humanity - just so he'd be able to swoop in in the last second and stop him. In the 'Bad' ending, he actually succeeds on the first part, and Mao invades and attacks the human world - but when Aurum tries to stop him, he unwittingly pushes Mao's Berserk Button by killing off his childhood friend, Raspberyl, sending Mao into a Suicidal Cosmic Temper Tantrum. So really, it might be considered an inversion of this trope, though matching nicely with the current name...
In Worm, Tattletale has a variation on this which she calls "Cops and Robbers", arguing that the superheroes are like football teams, and the less dangerous villains provide opposition for the public to cheer against.
A recent study of Carnegie Mellon University and University Of Pittsburgh students seems to indicate that honesty goes up when an "outsider" is said to have cheated and goes down when an "insider" is said to have cheated. This trope might even be Truth in Television! Not that Good needs evil, but that some of the paragons of Good might have come from refusing to associate with Evil traits.
In a different sense than good or evil, Nationalism didn't really kick off until Napoleon told Europe that they were French. They told him "nuts". Then sat down and thought about what they were, knowing what they weren't.
This trope is one of many, many theodicies presented as a response to the question of the "Problem of Evil", which asks "Why does an all-powerful, all-knowing, morally perfect God allow bad things to happen to good people?" Unfortunately the Problem is paradoxical in nature, so even if we assume evil is necessary, it still leaves 'gratuitous evil' unexplained. The idea that good is impotent without evil, does not maintain free-will, or builds character.