I apologise for the title. HN has a short limit for title length, so I came up with my own title. I thought this title did a decent job of presenting, using short language, the main application that the authors gave top-billing in the README.

I am not affiliated with the project.

JOSH claims to be reversible, so it could be used in either direction, which is where the multiple use cases come in. Treating subsets of a repo as their own repo, or treating multiple repos as one. I would say there is some application overlap between this and git submodule/subtree/subrepo and also tools like copybara.