Firestore method snapshotChanges() for collection

Question

Following is the code provided in Collections in AngularFirestore.

export class AppComponent {
  private shirtCollection: AngularFirestoreCollection<Shirt>;
  shirts: Observable<ShirtId[]>;
  constructor(private readonly afs: AngularFirestore) {
    this.shirtCollection = afs.collection<Shirt>('shirts');
    // .snapshotChanges() returns a DocumentChangeAction[], which contains
    // a lot of information about "what happened" with each change. If you want to
    // get the data and the id use the map operator.
    this.shirts = this.shirtCollection.snapshotChanges().map(actions => {
      return actions.map(a => {
        const data = a.payload.doc.data() as Shirt;
        const id = a.payload.doc.id;
        return { id, ...data };
      });
    });
  }
}

Here method snapshotChanges() returns observable of DocumentChangeAction[]. So why using a map to read it when it has only one array and it will loop only one time?