He must connect otherwise elements of "orphanhood" so that they can participate in the binding of data available, for example. view. This is especially important when you create a dialog box, for example. from the fragment, in the view controller. In the dialog box, the models that are installed on the view available to it will not be automatically installed ... but if you make the dialog dependent on the view, it will be.
See " Using dialogs defined as fragments " for more information.