alt Hacker NewsIt's for when you have a monorepo internally, and want to publish parts of it as open source to the world. They still need to live in the monorepo, so this is the solution.
Having a public repo as a dependency for your private corporate repo is a pain in the ass development-wise. Having a tree of such dependencies is a migraine.
It can also be used if you want part of your monorepo to track something open source from the world.
Say, to rebase upstream MySQL changes onto a fork in the monorepo (in a random, non-specific example)