Dependency Injection on Service in @Component metadata provider

I want to understand the 'Why' here. Most resources I checked online talk about how to Inject services in provider array of @Component metatdata and eventually in constructor.

But, I know that everything works fine if we do not mention is in provider array.

Then what difference does it make? Why should we or should not we opt mentioning it in the provider array? I am unable to find that answer.