Github is home to the largest number of open source repositories on the internet. There's a very good chance that all libraries that can be found elsewhere are also hosted on GitHub.
Many repositories are half finished, never made a release - or developer experiments.
So there are many projects to sort through when selecting projects.
Projects are listed with released versions, making it clear that the authors are releasing work they intend others to build on top of (unlike random repositories on 3rd pary hosting).