Angular 4: When and why is @Inject is used in constructor?

@Inject() is a manual mechanism for letting Angular know that a
parameter must be injected.

import { Component, Inject } from '@angular/core';
import { ChatWidget } from '../components/chat-widget';

@Component({
  selector: 'app-root',
  template: `Encryption: {{ encryption }}`
})
export class AppComponent {
  encryption = this.chatWidget.chatSocket.encryption;

  constructor(@Inject(ChatWidget) private chatWidget) { }
}

In the above we’ve asked for chatWidget to be the singleton Angular
associates with the class symbol ChatWidget by calling
@Inject(ChatWidget). It’s important to note that we’re using
ChatWidget for its typings and as a reference to its singleton.
We are not using ChatWidget to instantiate anything, Angular does
that for us behind the scenes

From https://angular-2-training-book.rangle.io/handout/di/angular2/inject_and_injectable.html