Very useful for local development. The documentation says it best:
"For development, a package can be linked into another project. This is often useful to test out new features or when trying to debug an issue in a package that manifests itself in another project."
When you're done, don't forget to run
yarn unlink on the child package (to
remove all symlinks) and
yarn install --force on the parent project (to
force the re-installation of dependencies and use the package from the project's
node_modules folder again).