Ah, I see. In that case, you're right, it's not: if you track issues and docs like like in the same way as you track source files, then functionality-wise they're no different.

The differences that remain are purely in the interface. For example, although they're synced the same way, VCSes tend to display issues differently than source; it's easier to filter out issue changes from the timeline, and harder to accidentally delete other people’s comments. It also makes it possible for people who aren't actual members of your project to raise and comment on the issues, which wouldn't usually be possible (unless your VCS can manage permissions for each file).

One particular use case of yours that Fossil does support is the ability to have a "wiki/" subfolder and have the web interface automatically format and display the pages inside, so you can link to them more easily if you're using the HTTP server for hosting. I think I use this feature more than the built-in wiki because it’s possible to version the docs along with the source, ensuring they never go out of sync.