The back rooms is specifically a creepy pasta from a few years ago.

The concept comes from “liminal spaces.” Which are areas in a building that exist to get from on place to another. Think stairwells in a school, office hallways, apartment lobbies, etc. Liminal spaces are weird because they only really exist to get us from one place to another. And because of that they tend to blend in with one another and all vaguely feel familiar yet void of other people. Like there’s one stairwell in my workplace that’s a very liminal space. It has a few paintings, fake plants, a stairwell, and nothing else. No one hangs out there for more than the thirty seconds spend getting from one floor to another. But it can be unsettling because this space is so bland it can be literally anywhere and so boring it’s meant to be forgotten and blends in with any other stairwell I’ve been in. So I guess the real horror of the back rooms comes from the feeling that it’s vaguely familiar because it’s like any other forgettable office anyone has ever been in.

Edit: Here’s an article on liminal spaces for a better explanation.

It's just a spooky internet story that random people add on to and expand. That's pretty much it. Someone thought of the idea, wrote a bit about it, and others have contributed their own ideas.

In any place that serves the public: hotels, shops, restaurants, bars, etc. there are front rooms and back rooms. The front rooms are the places accessible to everyone, where the customers usually do their business. The back rooms are only accessible by staff. These are rooms like offices, storage rooms, etc. The cultural concept of the back room is that any business conducted there is out of the public eye and therefore secret.

When 3D games are created, they generally involve using tools that let you create and move around objects, walls, etc. Creating each element one at a time would be very time consuming so once you create something it's easier to make a copy of it then change the copy slightly. Instead of making 1000 trees, you make four or five trees, and then copy them over and over. You can make a copy of a tree and then make changes to it, making it taller or wider, bending it, or whatever.

Oten a room or area is used to hold the original pieces. Instead of having to go find and load a tree, you just go to the room and copy the tree from there, then paste it wherever you want.

The same thing is true for rooms, buildings, characters, and so on.

It's sort of like having a box of premade bricks next to you when building something from Legos

The room or area where you keep this stuff isn't intended to be part of the game, but the devs want it available so they can make changes later if needed, so they make it an area you can only get to with the developer tools or cheat codes.

Players who use cheats or find other ways to get into these areas feel a cool sense of discovery that goes beyond what the game intended.

Developers know players will do this so they sometimes put fun things in these areas.

The game Portal really embraced this concept and made it part of the actual game itself, where you get to go behind the clean clinical walls of the environment and discover the back side of things, where the illusion is replaced by a more mundane reality.