Being able to run the same code on the client and server is a boon for web sites - rendering views server side then hooking up client events is a lot more performant than only rendering on the client side.

Also, context switching - the projects I work on tend to be front-end heavy, with a little API access and data storage on the backend. I find it a lot easier to do that backend work with the same language, tools and debugging environment as my client code.