What do ember server and ng serve really do?

I've read this Q&A:

What happens when you run ng serve?

It explains that when setting up an Angular 4 app using the official Angular quickstart steps (using Angular CLI to scaffold the Angular 'project'), and then running this command via the Angular CLI:

ng serve --open

... that what actually seems to be happening is that ng serve is doing the same as what the Ember server does, as it somehow 'piggybacks' on Ember. But what is the Angular server actually doing? This is JavaScript that will all be executed client-side. I'm not following why we need a server for Angular, and in what respect that the server is a 'server' at all. To be clear what I mean, why would we need a server to run the app in the client?? What is the Angular server really doing, and how should I think about this 'server' - I mean what is now our 'client'?