For example, you want to have a project in the monorepo, but also share it outside of your organisation, and even have external contributions brought back in.

Or, you are working in a project in the monorepo, but you don't want to (or can't) check out the entire repo. You can still checkout just that project and its deps.

Or, you are working in a polyrepo-using organisation, and you want to experiment with using a monorepo while continuing to allows devs to work in the polyrepos that it is composed from. No big-bang cutover necessary.