i think this is in part driven by the fact that javascript is not the best tool for the job on the client side, so people fall back on a variety of compile-to-j

i think this is in part driven by the fact that javascript is not the best tool for the job on the client side, so people fall back on a variety of compile-to-js languages. but then, once you're doing that, why not use a language you already feel is good enough to be your choice of server-side languages? the leap to "okay, now that we're using the same language for both parts of our app, why not make a framework that lets you mix server and client side code in the same codebase" is more a taking that idea to its conclusion than the primary motivation for it.