it's all about collaboration and source control.
"GitHub is a Git repository hosting service, but it adds many of its own features. While Git is a command line tool, GitHub provides a Web-based graphical interface. It also provides access control and several collaboration features, such as a wikis and basic task management tools for every project."
more info : https://techcrunch.com/2012/07/14/what-exactly-is-github-anyway/