For course creators to use the new community feature alongside courses, an additional level of page groups is required, to avoid the left pane navigation to become overcrowded with menu items.
Currently, you can create page groups in the very left side bar of the navigation in the community. But the option to create page groups within page groups does not exist (yet).
Picture this scenario: you have multiple courses which each have their own set of modules, e.g. 12 modules. Each module has its subset of content you want learners to take in sequence, e.g. a video, an audio-guided thing, a workbook, another video. So ideally, you want these elements as separate pages, not all stacked on one page. Once all elements of the module are completed, the next module is to be taken.
Currently, with page groups only available on the very left menu level, it would require the course modules to be on that level already, and as such, the left navigation bar risks become quickly very long and unorganised/confusing for the front end users, especially if you offer more than one course and you also want to use the community features for forum, discussions etc.