When developing and authoring multiple packages (private or public), you often find yourself in need of using the latest/WIP versions in other projects that you are working on in your local environment without publishing those packages to the remote registry. NPM and Yarn address this issue with a similar approach of symlinked packages (npm/yarn link). Though this may work in many cases, it often brings nasty constraints and problems with dependency resolution, symlink interoperability between file systems, etc.
yalc - npm