Collaboration and specifically collaboration with non git nerds. That's primarily what made GitHub win the VCS wars back in the day. The pull request model appealed to anyone who didn't want to learn crafting and emailing patches.
Yes, it's the PRs, and there is a misunderstanding I think because the OP and the GP's use-cases are quite different. Self-hosting your own repository on a remote server (and perhaps sharing it with 1 or 2 collaborators) is simple but quite different than running a public open source project that solicits contributions.
Yes, the projects that are emailing patches around generally have a much higher bar then the ones that accept GitHub PRs, but whatever works for a given project I guess
