We use Yarn because it allows us to more simply manage our node_modules. It also simplifies commands and increases speed when installing modules. Our teams module download time was cut in half after switching from NPM to Yarn. We now require all employees to use Yarn (to prevent errors with package-lock.json and yarn.lock).
We recently made the switch to Yarn mostly because of workspaces, speed, and the features in the Yarn v2 release. It has been great.