GitHub also drastically lowered the bar for contributing to free software. Contributing to a project used to mean subscribing to a mailing list, submitting a patch there, and not much would usually happen. With GitHub you fork, submit a PR, get your PR automatically built and unit tested with CI, the project owner can merge your PR with a single button click. It's fine to prefer an e-mail patch workflow, but GitHub has shown that for most of the population, submitting patches through PRs is a much lower bar.
At this point, GitHub is largely unnecessary. Most of the features have been copied and reimplemented better in FLOSS alternatives such as GitLab, Gitea, Gogs, etc. But since they were one of the first, people will stay due to network effects.