Why do I need a HTTP-server to run Angular 2?

I come from traditional back-end Java/Spring environment to learn Angular 2 framework, and have hard time to grasp many of the fundamental concepts.

I'm reading the Manning book Angular 2 Development with TypeScript and it says I need HTTP-server to run my SAP's. Why is that?

I thought Angular runs in client machine. So what is exactly function of the server? If I just open the HTML, Angular doesn't do it's magic.