Why shouldn't I provide services in components in Angular 2

Question

If I want to use service across the whole app, I should declare it as provider in app.module.ts, but it also works if it is declared in app.component.ts.

Is there any significant difference in those two approaches?

Answer 1

Let's say you have three components that are declared in your AppModule. If you provide service individually in every component, each of them will have its own instance of service, but if you provide it in AppModule, your service will be singleton - each component will share one same instance of that service.