Good Needs Evil

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.

Comic Books

 * 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.

Film
""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.""
 * The Fifth Element: Zorg's perfect example of the Broken Window Fallacy:

"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!"
 * Legend. Just before he's apparently destroyed by the light:

Literature
"Only in silence the word,
 * 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.
 * A chapter quote from Ursula K Le Guin's A Wizard of Earthsea.

only in dark the light,

only in dying life:

bright the hawk's flight on the empty sky."

- From "The Creation of Ea", an Earthsea poem


 * A major theme in Paradise Lost
 * Some of the Dragonlance books have a darker take on this. The god of good in this setting is a Well-Intentioned Extremist with impossibly high standards, and he'd like to see all villains redeemed (through brainwashing if necessary), or else slain. If he actually beat the forces of evil, he'd become just as bad.
 * In a nutshell, good needs evil to have something to fight against. If no outside targets are forthcoming, some of the good becomes evil to fight against itself.
 * This is the entire point of Villains By Necessity.
 * It underlies The Lord of the Rings and the rest of Tolkien's world.

Religion and Mythology
"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?"
 * Defied by the apostle Paul in The Bible, against people who rationalized this as an excuse to keep doing things they know are sinful:


 * 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.

Video Games

 * Disgaea 3.  So really, it might be considered an inversion of this trope, though matching nicely with the current name...

Webcomics

 * This Sinfest strip, spoofing Mars Needs Women.
 * This is a major part of Tarquin's argument as to why it's a good thing that he's evil in Order of the Stick.

Web Original

 * In Interviewing Leather, Leather asserts this philosophy.

Western Animation

 * During Satan's song in the South Park movie, he sings "without evil there could be no good, so it must be good to be evil sometimes".