The singleton pattern is the 'Superpower' of Angular. It allows you to create a centralized state that acts as the single source of truth for your entire application.
1State Persistence
One of the biggest challenges in SPA (Single Page Application) development is keeping data alive while the user moves around. Since components are destroyed and recreated during navigation, you cannot store long-term data inside them. Singleton services live as long as the application is running. By moving your application state into a service, you ensure that the user's progress is never lost as they move through your routes.
2The Observable Service Pattern
While a simple property in a service can store data, it isn't 'reactive'. Components would have to constantly poll the service to see if the value changed. By using a BehaviorSubject, you turn your singleton into a broadcaster. Components 'subscribe' to the data stream and are automatically notified (and updated) the millisecond the service data changes. This is the foundation of high-performance Angular UI.
