All of these features cooperate and serve the same goal: coordinate the work product of people on a project, in a distributed fashion. One path to that is the nearly fully centralized model of GitHub. Another is the VCS + mailing list + bug tracker + wiki path, which requires considerable admin resources to manage, and at the end of the day is a pile of barely-cooperating services. Fossil's path is to put them all into one place so they all work properly together.
You can reference ticket IDs from a forum post.
You can point to a section of the timeline from a wiki article.
You can create diagrams in Pikchr format that live as version-controlled text in the repo and reference them from commit messages.
You can generate HTML diffs and include them into the body of a Markdown chat posting for discussion of a proposed change before committing it.
Etc., etc. It's all communication, which you need when you have multiple people working on a project, especially across time zones.