It switches the axis of concern from separating out the different classes of code (HTML, JS, CSS) to separating out the functional goals of your code (Component

It switches the axis of concern from separating out the different classes of code (HTML, JS, CSS) to separating out the functional goals of your code (Component A does this specific thing, Component B does that specific thing). Realistically, the triad of front end technologies are tightly coupled, regardless of whether they're in the same file. If you can keep your components nicely bounded, then it's far easier to have them be self-contained.