Every person who is intelligent enough to be self-aware, and that is over ninety percent of all people in spite of Fox News and various extreme religions, has a Dark Side that they are aware of.
And most people are sensible enough to show off the Light Side and keep the Dark Side hidden.
Only fools and geniuses reveal the Dark Side and play down the Light.
I consider myself a fool. Decadent poets like Charles Baudelaire and Paul Verlaine are examples of the geniuses.
Baudelaire himself believed that, “The way down is the way out.” Meaning, I suppose, that you can only be at peace with your personal demons at the bottom of the liquor bottle, the sharp end of the cocaine needle, or the grave.
I myself don’t put the demons forward in drinking or taking drugs. And I am definitely not trying to die young. Instead, I grapple with the Dark Side in fiction where I can kill it with a silver bullet, or pull it down into a pit of computer glitches where it will delete itself.
My personal darkness comes from traumatic experiences in my youth and childhood. I was sexually assaulted as a child and kept it secret for years. I grappled with suicidal thoughts and self harm as a teenager.
So, why am I now thinking about the darkness again?
Well, one of my books is in the process of being read and reviewed at this moment. It is the Baby Werewolf, a book in which I take on the darkness of feeling like I am a monster and only worthy to live in darkness. The story reaches its climax with the firing of a silver bullet.
I wish it was as easy as firing a silver bullet to deal with the Dark Side. It is not. I have fired dozens. Some monsters of the mind are purely bulletproof,
Still, some of my best work only comes about due to the Dark Side. And writing about it is the only way I can control the madness.
Catch a Falling Star 
Demons are built in drives that either derive from overpowering instincts or traumatic life experiences that threaten to take control of your life and potentially ruin it.
I found it best to accept my demons as an inescapable part of me and even to befriend them. Find safe places for them to play where they cannot hurt anyone. Sometimes they are even useful in getting by the harsher parts of life, like my “FTW!” demon. Or my “I wanna run around naked!” demon. My super-logical “Mr. Spock” Asperger’s demon. My “Dirty Harry” demon. (Go ahead. Make my day, punk!)
Sometimes I can use one demon to fend off another. Like the first time I ever modeled for an art class I used the “FTW!” demon to fight off my “panic” demon.
Your demons are not always evil nor are they always wrong. But out of place they CAN make a mess of things.
The key is to let them know they are passengers on the bus. I am the driver and decide when and where where the bus stops and who gets to ride at the moment. That’s a tough thing to do. But you can’t suppress demons all the time. They will find a way out. It is best that one exercises some control over time and manner.
I guess you are right about the function of demons. I use them in fiction where I can wrestle with them, flip them around, and sometimes shoot them with silver bullets. But I am not sorry that they are there to use as I need them.