It exists. It affects people's lives. But the story never focuses on investigating its origin or how it works, only someone whose life is affected by it
Sorry for ambiguous title. It's basically The Unreveal/Riddle for the Age/Nothing is Scarier mixed with a kind of storytelling in which the setting has something that affects people's life, and the narrative is about one of those people.
The One Wish Willow in Obsession
Somehow it's a common toy in the movie's universe. You make a wish and break it in half, and your wish will surely come true; however, one person can only wish once. The movie isn't about investigating how these things are made or how it really works, but about a young man's wish gone wrong.
The Substance in, well, The Substance
Another popular item, the Substance is a strange liquid that, when injected into your body, will create a younger version of you that tears your back apart and comes out; from then on, you have to regularly switch back and forth between the two bodies and use the original one's "stabilizer fluid" to maintain the new one. Again, the whole movie only follows a woman who uses this and how things go wrong.
The Death-Cast in They Both Die at the End
In this novel's world, there's a service called "Death-Cast" that calls people to inform that they only have about 1 more day to live. Apparently, its forecast is so correct that nobody bothers fighting it, and the story simply follows two young men who get its call on the same day, as well as their friends.
The email that makes you invisible in Toumei Ningen no Tsukurikata
An early day manga short series from Masuda Eiji (author of Jitsu wa Watashi wa), it follows a high school boy who one day receives an email that say "You only need to reply to this email and you'll become invisible to the world"; apparently it doesn't matter what you type in reply, and you can also resend it to someone else to coax them into become invisible too. The whole story is about the boy's horror as he replies and discovers that "turning invisible" means that his existence is erased from the world's perception. It's never shown if anyone has ever tried to investigate this email, and while a group of invisible people (aside from the antagonist) appears in the final chapter, it looks like they just drift aimlessly around the world fending for themselves.
The store that buys your lifespan, time, or health in Three Days of Happiness
In this novel/manga, there's a store that you can come to sell lifespan, time, or health if you're in need of money. If you sell lifespan, you only get to live out the time you haven't sold; if you sell time, you'll be invisible to most the world during the time sold, only able to be seen by and interact with the store's other employees and customers (in other word, you become a temp worker for them in a roundabout way). It's never explained how the store works nor how it kills the people that sell lifespan, aside from the implication that they die naturally in their sleep; we only ever follows the last days of a young man who chooses to sell lifespan. (Also, it's never explained what happens to those who sell health).