What is the purpose of providedIn with the Injectable decorator when generating Services in Angular 6?

providedIn: 'root' is the easiest and most efficient way to provide services since Angular 6:

1. The service will be available application wide as a singleton with no need to add it to a module's providers array (like Angular <= 5).
2. If the service is only used within a lazy loaded module it will be lazy loaded with that module
3. If it is never used it will not be contained in the build (tree shaked).

For further informations consider reading the documentation and NgModule FAQs

Btw:

1. If you don't want a application-wide singleton use the provider's array of a component instead.
2. If you want to limit the scope so no other developer will ever use your service outside of a particular module, use the providers array of NgModule instead.

if you use providedIn, the injectable is registered as a provider of the Module without adding it to the providers of the module.

From Docs

The service itself is a class that the CLI generated and that's decorated with @Injectable. By default, this decorator is configured with a providedIn property, which creates a provider for the service. In this case, providedIn: 'root' specifies that the service should be provided in the root injector.

From Docs

What is Injectable decorator?

Marks a class as available to Injector for creation.

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class UserService {
}

The service itself is a class that the CLI generated and that's decorated with @Injectable().

What exactly does providedIn do?

Determines which injectors will provide the injectable, by either associating it with an @NgModule or other InjectorType, or by specifying that this injectable should be provided in the 'root' injector, which will be the application-level injector in most apps.

providedIn: Type<any> | 'root' | null

providedIn: 'root'

When you provide the service at the root level, Angular creates a single, shared instance of service and injects it into any class that asks for it. Registering the provider in the @Injectable() metadata also allows Angular to optimize an app by removing the service from the compiled app if it isn't used.

providedIn: Module

It's also possible to specify that a service should be provided in a particular @NgModule. For example, if you don't want a service to be available to applications unless they import a module you've created, you can specify that the service should be provided in the module

import { Injectable } from '@angular/core';
import { UserModule } from './user.module';

@Injectable({
  providedIn: UserModule,
})
export class UserService {
}

This method is preferred because it enables Tree-shaking (Tree shaking is a step in a build process that removes unused code from a code base) of the service if nothing injects it.

If it's not possible to specify in the service which module should provide it, you can also declare a provider for the service within the module:

import { NgModule } from '@angular/core';
import { UserService } from './user.service';

@NgModule({
  providers: [UserService],
})
export class UserModule {
}