Angular2: return type of function when implementing APP_INITIALIZER

Question

I have created an Angular 12 Application to learn how APP_INITIALIZER works. Here is an extract of its app.module.ts

function initializeApp(appConfigService: AppConfigService) {
  return () => appConfigService.load();
}

@NgModule({
  declarations: [AppComponent, MyComponentComponent],
  imports: [BrowserModule, HttpClientModule, FormsModule],
  providers: [
    AppInitService,
    {
      provide: APP_INITIALIZER,
      useFactory: initializeApp,
      deps: [AppConfigService],
      multi: true,
    },
    {
      provide: APP_INITIALIZER,
      useFactory: initializeApp1,
      deps: [AppInitService],
      multi: true,
    },
    {
      provide: APP_INITIALIZER,
      useFactory: initializeApp2,
      multi: true,
    },
  ],
  bootstrap: [AppComponent],
})
export class AppModule {}

The app is up and running with appConfigService being called before the app is ,,running". However, I am not very sure why in the initializeApp() function I have to return a function (otherwise the app would throw errors like below):

core.mjs:6484 ERROR TypeError: this.appInits[i] is not a function
    at ApplicationInitStatus.runInitializers (core.mjs:24748:36)
    at core.mjs:26040:28
    at _callAndReportToErrorHandler (core.mjs:26143:24)
    at core.mjs:26038:20
    at _ZoneDelegate.invoke (zone.js:372:1)
    at Object.onInvoke (core.mjs:25476:33)
    at _ZoneDelegate.invoke (zone.js:371:1)
    at Zone.run (zone.js:134:1)
    at NgZone.run (core.mjs:25330:28)
    at PlatformRef.bootstrapModuleFactory (core.mjs:26017:23)

My question is, that why I need a callback function in my initialzieApp() and cannot instead return the result of appConfigService.load() directly.