As the amount of pages and frames grows in complicated projects with multiple designers working with it (like information systems), including conceptual mockups, interactive prototypes, variants of design solutions, states of behavior for developers and so on, it becomes a mess to organize the whole project in a flat page structure.
In past we used another prototyping tool, that helped to organize all the mess with folders and pages-subpages hierarchy. So it would be nice to have something like this:
Probably there are other solutions, but keeping separate projects for interactive demo prototypes, states of the specific page’s behavior, etc. was ineffective attempt that we tried.