Stand behind a solid wall in a game. The room on the other side may still exist, contain characters and respond to game logic, even though the renderer can avoid drawing some of it. Occlusion culling is one way to make that distinction useful.
The renderer asks whether an object is hidden by other geometry from the current viewpoint. If it can safely omit the object, it can save drawing work. This differs from frustum culling, which removes objects outside the camera’s viewing volume. An object can be inside that volume and still be completely behind a wall.
Unity’s conceptual explanation uses connected rooms to illustrate the benefit. That page describes an older engine version, so its interface details should not be treated as current setup instructions. The underlying visibility problem remains a useful way to understand the technique.
What the optimisation does not decide
Not drawing a character does not inherently stop its AI, physics or sound. Those systems may have their own rules for distance and activity. A game can therefore let an unseen enemy move while omitting its visible model, or deliberately simplify several systems together. The result depends on the implementation.
Consider a doorway in a corridor. When it is closed, much of the room beyond may be hidden. When it opens, some geometry becomes visible. The system needs information that matches that situation; treating every obstruction as permanently solid can produce objects that disappear when they should be seen.
Unity’s current troubleshooting guidance describes conservative visibility decisions and the trade-off between data detail, memory and processing. Skipping more objects is not automatically free.
Read a visual glitch cautiously
If a building briefly vanishes as the camera turns, occlusion is one possible area for developers to investigate. Streaming, level-of-detail transitions and other rendering systems can produce superficially similar symptoms. A player cannot identify the exact cause from one screenshot.
A useful bug report records the location, viewpoint, movement and whether the problem repeats. A short clip showing the approach is more informative than a close-up taken after the object has disappeared.
Occlusion culling is successful when you do not notice it: the visible scene stays coherent while unnecessary drawing work is reduced. The hidden world and the rendered image are related, but they are not the same workload.