The use case is right there in the text:

> enabling unmodified programs expecting a Unix-like environment to run directly in the browser

The latex example is probably the best example. There are a handful of latex typesetter web pages, but the "quick and dirty" implementation for all of them are something like:

(1) take the input (2) shell out to run the latex app on a unix box (3) return the output to the user via the browser

Step (2) requires a complete unix environment. As usage scales, so too does the processing power required. Imagine if that app got 10k requests/second: suddenly you have a farm of "rendering servers" and middleware managing availability, request routing, etc.

This allows you to offload step (2) to the user's browser for a broader range of applications, with easier/faster/less rewriting. While the upfront cost is still more complex if you have to port the app to this js platform, there may be long term savings in simplicity/efficiency from the reduced server requirements. e.g., you could reduce the latex typesetting webapp to static files served from S3.