People writing service-side “back-end” code have many options on technologies to use. There are multiple web application platforms that are built around different languages. Ruby on Rails and Sinatra, Django and Flask, PHP, Node.JS, the list goes on.
No web developer is expected to hand-write WebAssembly on a regular basis. But once WebAssembly adoption takes hold across the major browsers (and it seems to be making good progress) it opens up the field of front-end code. Google is unlikely to build TypeScript into Chrome. Microsoft is unlikely to build Dart into Edge. Mozilla is not going to bother with CoffeeScript. But if they all agree on supporting WebAssembly, all of those languages – and more – can be built on top of WebAssembly.
The concept can be taken beyond individual programming languages to entire application frameworks. One demonstration of WebAssembly’s potential runs the Unity 3D game engine, purportedly with smaller download size and faster startup than the previous asm.js implementation.