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.