If you've ever worked in other languages that have an "import" function, you'll understand. Having to manually manage script tags, order of operations, rapidly growing 2000 loc script.js files, etc, can only lead you astray, introduce errors, and make debugging your code hell.

// App Controller

var AppModel = require('./models/AppModel');

var NavView = require('./views/NavView');

var HomeView = require('./views/HomeView');

// Do stuff

You can immediately know whats going on inside of your class or module simply by looking at the imports, the LOC is greatly reduced, and refactoring, via imports, makes your code about a thousand times more maintainable.