Sometimes you just clone a repository to experiment with the project’s code and don’t know in advance whether you’ll be sending pull requests. You do:
git clone git@github.com:user/project.git
cd project
# and try to compile or run code
Then, at some point, when you find a bug or just want to add new functionality, you fork the repository on GitHub. And instead of cloning your newly created fork into a new folder and manually transferring your changes, you can run these commands and continue working in the same folder you cloned the original repository into:
git remote rename origin upstream
git remote add origin git@github.com:you-user/project.git
Then you create a branch, commit your edits, and send a pull request.