In "The Sandman", what are nightmares for?

Question

What purpose do nightmares serve in the world of The Sandman? In the second volume of The Sandman Library (The Doll's House), Dream spends most of his time tracking down and dealing with a number of errant nightmares (Brute, Glob, and the Corinthian). He refers to these as "his creations" many times, unforming some (to be reformed later) and banishing others (for a few thousand years, implying that they will eventually return).

What purpose do these creatures serve? Dream seems to take his "job"¹ very seriously; this seems to include caring for the well-being of sentient creatures -- the other main story of The Doll's House, for example, was that of the dream vortex, which posed a great danger to anything that dreamed, and Morpheus did his best to avert the impending disaster. In this volume, he also disbands and attempts to rehabilitate the Collectors.

One could argue that it's possible that while nightmares are not good for individual humans, they serve a greater purpose for humanity. The basis for that argument would be that the focus of Morpheus' care for sentient creatures appears to undergo some shift from humanity-only to include individual humans, and the nightmares were created long before Dream cared much for lone individuals. The only caveat that accompanies this interpretation is the apparent absence of a benefit for humanity in general. (At least, I don't see one.)

It cannot either be that nightmares are things that humans (or sentient beings) find in their minds, and that Dream does not remove -- in this universe, nightmares are the creations of the King of Dreams. They are things that he expressly created, for some purpose that escapes my understanding at the moment.

Do they serve some purpose in-universe? Are they just a plot device, or a metaphor? (Something else?)

When I searched Google, I was not able to find much about nightmares. The shared Wikia page for dreams and nightmares says (without a source) that "Nightmares are beings created by Dream to populate his realm and to help him with various tasks." This does not, however, explain what the purpose of nightmares is. TV Tropes seems to treat them all as mere "Nightmare Fuel," but I think there should be a greater purpose in them, especially since Neil Gaiman is such a careful writer, and Dream is such a careful guardian.

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¹ I'm not sure what, exactly, that job is. In three volumes of The Sandman, I have yet to see a job description. Perhaps that will come later.

Answer 1

The Dreaming is supposed to be a reflection of the real world... or maybe vice versa? In any case, The Dreaming (and Dream), in a sense, define the real world - by defining the things that are not real.

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According to Destiny, Morpheus' realm is a "metaphor" or an "allusion" of the real world; it's then easy to interpret dreams dreamers' reflections on the real world. It's not like dreams are defined as binding scenarios in The Sandman - they're more like stereotypes, or actors, used to convey a message.

What message could a nightmare convey then? Corinthian, according to Morpheus, is supposed to show dreamers the "darkness inside them":

Morpheus described Corinthian in more detail in The Kindly Ones:

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Clearly, Corinthian was intended to show people that they, of everyone else, are their own antagonists.

How could one convey that message in a nice, happy dream? I imagine this method is the most powerful - how to convey the horrors one carries better than with a horror?

Nightmares, then, are intended for metaphors and allusions that require this, and not some other approach.

In the same way, good and cheerful dreams are supposed to reflect the happiness of the dreamers. If I was a sardonic person, I'd indicate that some people take joy from the suffering of others, but that's another conversation.

I'm saying "reflect" because it's what is implied throughout the whole story - that the Endless do not control but execute the will of the living beings. Hob Gadling's story may be a mockery of that - Death will not take him because he doesn't accept death as a part of life. Morpheus himself says that "[the Endless] are [living beings'] puppets":

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Answer 2

You've asked the question from the wrong angle, so let me answer from the end:

What is Dream's job description?

Dream's job is to maintain a dreamland - the place where people go when they are dreaming. If he is not maintaining it, the dreamland tends to run wild, which can cause weird symptoms on Earth - the "sleeping sickness" that was running when Dream was imprisoned. He is also creating dreams and nightmares, and he is responsible for killing dream vortexes.

So why not making only nice dreams?

Well, the quickest answer would be "because Morpheus is not always a nice person" (he is, for most of the time quite indifferent) - he sentenced a girl that spurned him for ten thousand years in Hell and more than once he was happy to leave people for certain death. His morality is quite different to human - in other words he doesn't necessary find any difference between a pleasant dream and nightmare, all he cares about the quality of the provided dream. And in some ways nightmares must be more precise and robust, because they are universal - we all occasionally have a visit from Corinthian, when we dream about slasher-killer chasing us through the darkness.

Also, as a human beings we DO NEED to have nightmares:

Nightmares are helpful to our survival or else they probably would have been done away with by evolution, said Deirdre Barrett, a psychologist at Harvard University. Barrett theorizes that nightmares act as the brain's way of focusing a person's attention on issues they need to address. "Nightmares probably evolved to help make us anxious about potential dangers," Barrett said. "Even post-traumatic nightmares, which just re-traumatize us, may have been useful in ancestral times when a wild animal that had attacked you, or a rival tribe that had invaded might well be likely to come back."

From the behaviour of Corinthian , we can also assume that nightmares can work independently as sort of field agents and are able to affect the mortal world in much more direct way than Dream can (or want): Corinthian was a serial killer for years, later he works as a private eye (no pun intended).