The biggest advantages monorepos have offered is development of tools like lerna(1) or yarn workspaces.
Before that there used to be a node_modules folder with GBs of [useless] data in all my projects. Now there is just one folder on top and that's it. Also if you're developing lots of modules or plugins it makes it super to work without committing changes since they are symlinked.
node_modules is an interesting worst case of package management systems.
There's some good exploratory work currently happening on making node_modules and the node package ecosystem better in general, but especially in the polyrepo case. Yarn "Plug'n'Play" is one, and Tink [1] the other.
Before that there used to be a node_modules folder with GBs of [useless] data in all my projects. Now there is just one folder on top and that's it. Also if you're developing lots of modules or plugins it makes it super to work without committing changes since they are symlinked.
(1) https://lernajs.io