In a repo with unstaged changes, running "git checkout ." will checkout files from a specific commit into the worktree and clobber your unstaged changes. By default it probably uses HEAD as the commit to checkout from.

You can checkout files and directories from completely unrelated commits and even unrelated repos!

I have used this to checkout specific useful files from other projects. It's a little nicer than just copying them in, because you can keep the repo tracking branch updated and keep checking out updates, and you can easily compare your file to theirs, and see what changes they have made to the file.