A nice use case for portals (other than modals, dropdowns etc.) could be dynamic widgets on websites (like the typical wordpress / drupal websites). For example, if you have dynamic widgets which are spread across the page, but somehow belong together. So you could have a widget for the search-field in the header of the page and a widget for the search-results in the content area and both would be part of the same react app. Or a table with filters, where the table and the filters are two separate widgets.