Instead of restating the details, we will paraphrase directly from Angular's official documentation (https://angular.io/guide/ngmodule-faq#!#q-why-child-injector), which explains this perfectly:
Angular adds @NgModule.providers to the application root injector unless the module is lazy loaded. For a lazy-loaded module, Angular creates a child injector and adds the module's providers to the child injector.
This means that a module behaves differently depending on whether it's loaded during application start or lazily loaded later. Neglecting that difference can lead to adverse consequences.
Why doesn't Angular add lazy-loaded providers to the app root injector as it does ...
