One of the fundamental differences between SVN and Git is that Subversion is Centralized. That means there must be a centralized master authority with respect to the codebase.
As you are seeing while you work on Open Source projects, a Distributed SCM - like Git - does not necessarily require one centralize repository. Each user has a complete copy of the entire history and can work independently. Changes can be later merged into those of another developer (GitHub uses the Pull Request as a means to 'request' your changes are included but provides the maintainer ultimate purview over the project). This can even be from one machine to another! Changes (in the form of patches) can be sent via email to be merged into a copy of the Git repository held by another developer.
So, overall the philosophy is "I can work on MY code, protect MY code from others, share MY code as I desire" without the necessity of connection to a master centralized data store. In practice, particularly in the Open Source world, there is a repository understood to be that MASTER. And Git supports that concept easily. This is what has made GitHub so large.
I may not have answered your question directly - about Workflow and Use Cases - but I hope this provides a clearer perspective on how Git "thinks". There may be others here who can offer Case-History stories about particular workflow(s).