Scenes can get pretty busy sometimes, especially when we're building large, open worlds. The more objects there are invoking code in an Update() callback, the worse it will scale and the slower the game becomes. However, much of what is being processed may be completely unnecessary if it is outside of the player's view or simply too far away to matter. This may not be a possibility in large city-building simulation games, where the entire simulation must be processed at all times, but it is often possible in first-person and racing games since the player is wandering around a large expansive area, where non-visible objects can be temporarily disabled without having any noticeable effect on gameplay.