A number of things - do you have a small microservice that was easier to write as server-side code, but you don't want to worry about scaling it? Run it in-browser without code changes or worrying about jank - the server is run in a Web Worker, and any long running computations can happen concurrently with rending, layout, and input handling.

Same with traditional C and C++ applications. Wrapping program invocations or APIs with Emscripten can be a pain, and if you have a couple of programs you want to use together, forget it. Browsix lets you compile the program down to a single js file, provide input as you expect (by sticking files on a filesystem, or providing stdin), and register a handle for stdout + stderr.