The issue that Fossil addresses from my perspective is the integration of bug/issue tracking and the VCS. The trouble with using Git{hub,lab} or other issue tracking is that the issue control isn't distributed in the same way as the source control is.

I want to be able to do a code review, deal with an issue, make changes to source, test etc offline/on a branch and then push all of the changes/impacts (source, issue, comments etc) for distribution to other developers.