It’s basically like a copy of the original repository. But you can pull in and merge changes from the original, make a pull request for the original to pull your changes. Fork+pull request enables you to contribute to someone else’s repository. Things like Chromium are in part forks of Safari, just that they diverged over time.
Well LoL has no official Linux support, so a low current number of users is no indication of the size of the potential Linux player base.