When you render a child component you are implicitly defining a tree of components. #children simply allows the framework to walk the component tree explicitly. The reasons for needing to walk the tree explicitly have changed over time, which is part of the reason for the confusion.
At one point, for example, we used to walk the tree to give each component a chance to handle callbacks, so if your component wasn't in #children it would never even have seen its callbacks. That is no longer the case (which is actually a bit of a shame because decorations can no longer intercept them, but I digress).
If you look in the image for users of WAVisiblePresenterGuide and WAAllPresenterGuide, you will see the current cases where we need to traverse the tree:
- Calling #updateStates: for snapshotting/backtracking
- Calling #initialRequest: when a new session is started
- Executing tasks (they need to execute outside of the render phase to make sure the render phase does not have side effects)
- Calling #updateRoot:
- Calling #updateUrl:
- Displaying Halos for each component in development mode
- Generating the navigation path in WATree
- Detecting which components are visible/active to support delegation (#call:/#show:)
- the framework may change in the future to traverse the tree for other reasons;
- add-ons may depend on being able to walk the tree for other reasons; and
- it's not great encapsulation to assume that, in the future, components you are rendering will never need any of the above nor start using sub-components that do.