app.module.ts中的注册服务与angular4中的单个组件有什么好处？

app.module.ts

import { AppComponent } from './app.component';
import { MyComponent } from './mycomponent/mycomponent.component';
import { MyService } from './services/myservice.service';


@NgModule({
  declarations: [
    AppComponent,MyComponent
  ],imports: [
  ],providers: [MyService],bootstrap: [AppComponent]
})

mycomponent.component.ts

import { Component,OnInit } from '@angular/core';
import { MyService } from '../services/myservice.service';

@Component({
    selector: 'my-selector',templateUrl: './mycomponent.component.html',styleUrls: ['./my-component.component.scss']
})
export class MyComponent implements OnInit {

    constructor(private _myService: MyService) { }

    ngOnInit() {
        this._myService.test()
    }

}

但我也可以在组件本身注册使用它,而无需向app.module.ts添加任何内容

import { Component,styleUrls: ['./my-component.component.scss'],providers: [MyService]
})
export class MyComponent implements OnInit {

    constructor(private _myService: MyService) { }

    ngOnInit() {
        this._myService.test()
    }

}

我很好奇,每种方式有什么不同？