Discoverability. Without federation, people are pretty much dependant on GitHub to make sure their software gets out there.

How do you discover new software using GitHub? Let's say I want an RSS reader for Linux - how does GitHub help me find one? I must have never used this part of GitHub.

Github has search functionality and grouping of repos by topic, etc. So you can browse repos related to a specific topic. Or you can click on someone's profile and see the projects they've worked on and maybe one of them is interesting.

Github ranks higher on Google search.