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.
I feel that the additional layers of indirection are quite costly for the convenience of running programs in a Unix environment inside the browser. I can understand the practicality of that for prototyping, but it seems a bit baroque for a production system.
But then, the idea of heaping Javascript monsters upon a mere browser seemed ludicrous to me 20 years ago - so I may not be the best judge of those sort of things...
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.