A frequent use case imho is the need to quickly view the global page hierarchy of a wiki, especially all visible nodes as you point out Vincent Massol: this significantly improves content discoverability and simplifies navigation.
I’ve noticed that XWiki’s competitors don’t seem to offer this feature out of the box though. Also, the Eclipse documentation has a "Collapse all" button, but no "Expand all".
I'm adding below a script that expands the visible nodes (can be used from the browser console). +1 for adding this feature by default, with an option in the Navigation Panel configuration to enable or disable the button?