If you maintain OSS projects, this can be a useful tool to communicate that you are working on a particular, often-requested feature. Or perhaps, that you are on vacation, in hiatus, or sick.
Likewise, this could be a useful feature in the context of enterprise GitHub or private repos used by organizations. Seeing the current status of a coworker without leaving GitHub is a nice plus, especially if they start integrating statuses with tag autocompletion.
Now if they just evolve their "discussions" (formerly issues) into "live discussions", and then add one more feature to allow separating it by "tag" or maybe "topic" per repo, they can pretty much replace Slack.