>With javascript in the browser you can see and experience code-reload after an F5†. There is no compilation time; no real detriment to importing another non-used library if it's not compiled into your code.

While that may be true, typical JavaScript projects require a compilation step. Plain JavaScript is starting to get somewhat rare, and projects typically use a compile-to-js language like CoffeeScript or TypeScript, or perhaps ES6-to-ES5 compile step. And then you need to minify that code since it's not recommended to run a non-minified "debug" version during development. And if you're project is any complex you're probably also compiling a single .css file perhaps from .less, compiling .html from .jade and packaging it all up in a different public folder.

The point is that typical non-trivial web applications need a build step.