In some cases a controller desires to open some action in a new window, e.g. to open a document editor.
The problem is that the URL of that call must open in a new window and thus must also create a new window object on the server, but at the same time the controller who offers this link must be notified and must offer the code to generate the content / controller hierarchy of the new window. In the example above, the controller must provide the path to the document, check if the user is allowed to edit etc.
The framework must be extended in a way that this communication is possible, either by events, callback or some other techniques.
An important aspect of the feature is that the initiating controller can not generate the window object in advance. Imagine a directory with thousands of files, all with an edit link. The edit link must be a normal link that can be opened with target=_blank and still contain enough information so that the context can be restored and build in the new window.