People on GitHub is not just to sharing codes, but also to get in touch with each others. GitHub succeed because it's not only just a free online Git repo services, but also a developer + user community where you can put your code on, share it, and 'earn' stars & forks as feedback. And stars + forks can help you stand out in a job interview and many other occasions.
Bitbucket is another Git repo service, but it sort of failed to build it's community. Result? It received less attentions compare GitHub.
So, while GitLab is also trying to be 'yet-another' Git repo + you can host it on your own, the benefits of become community can't be ignored. And federation can help that by connecting all the GitLab instances together to form a bigger and global community.
Even better, the federation protocol itself can be an open-sourced public standard, so all the other Git repo software can implement that in their product. The potential is huge.