It's the broadest level at which two elements can affect each other.

Classic example: if you have a notification bubble near the top of the page, and a notification gets created by some component hidden deep inside the app structure, that message can't be handled in a local sub-model; it must travel all the way to the root node to update the notification bubble stack in the main model.

(Which is why nowadays I prefer to skip the nesting and just have a flat list of all messages in the app. With good spacing and naming, it's quite maintainable and just as solid.)