I worked on a robust client-side web application built in React. There was a product decision to make public one of the main pages with less information than a paying logged-in user would see. It was fairly straight forward, if somewhat of a quick hack, to do a server-side rendering of that page, cache it in Redis and expose it to the public web (along with building sitemaps and publishing those). It resulted in the company showing up in search results a lot more thus positively impacting growth.

I hesitate to hold this example up as a shining beacon of the great power of using the same language on server and client. There were a fair number of pros and cons. But the end result was very useful and execution was relatively fast.