What role does the @Injectable() annotation play?

@Injectable() tells Angular that dependencies can be injected into this class and that it can itself be injected via DI.

Simply put:

• Without @Injectable(), Angular cannot create an instance if the class has dependencies inside it.
• Through the providedIn parameter (e.g. 'root') you can specify exactly where this service will be available: at the root of the application, in a module, or in a component.

So @Injectable() makes the class part of the Dependency Injection system.