I've found two major benefits with yarn. First is package install time, like others have said it's an order of magnitude faster for projects I've worked on.

Second is a little more niche in running a Linux image on Windows host via Vagrant/Virtualbox. Out of the box symlinks won't work so bin files often fail. I would also run into file locks when running npm install on larger projects often, to the point where I was having to run it 5-10x before everything would finish installing. You can instruct Vagrant to offer some support (setextradata VBoxInternal2/SharedFoldersEnableSymlinksCreate/v-root 1), or set an environment variable to key on and update any scripts to use --no-bin-links, but it's flaky in my experience. I never found a solution to the file locks with npm.

I installed yarn on a whim on that setup and have yet to have any of those issues with it.