Issue trackers and discussions on FOSS projects are effectively a public forum, and on any public forum people start building brands and identities. Add comment reactions and GitHub stars into the mix and you have a vicarious karma system whereby people's projects jostle in popularity and people try to build reputations. For some people, GitHub is more of a social network than a VCS tool.
GitHub is also somewhat entangled with junior hiring in compsci, meaning there's a tangible economic incentive in building a hireable profile that a recruiter might like. (Or at least that is a widespread belief.)
None of this has much to do with git, the VCS tool, nor is GH-as-a-social-network objectively very important, but it's natural to place outsized importance on a community in which one is active, and there's a significant overlap in the HN commentariat and GH users. So you'll probably hear more about the claimed social utility of GH here than in the real world.
Hope this helps clarify what people are talking about. :)
