One of PyMiniRacer author speaking.

Use cases are many. For instance, Rails uses ExecJS to minify assets, to compile JavaScript assets from ES6 to older JS runtimes using Babel, for compiling CoffeeScript... In another world - and that's Sqreen use case for this, JavaScript is the 2016 alternative to Lua.

Furthermore, PyMiniRacer is a lightweight alternative to NodeJS if you are a pythonist and need to run JavaScript, for production or for hacking :)

And server side rendering :)