What is the role of NG_VALUE_ACCESSOR in a custom component?

import { Component, forwardRef } from '@angular/core';
import { NG_VALUE_ACCESSOR } from '@angular/forms';

@Component({
  selector: 'app-custom-input',
  providers: [
    {
      provide: NG_VALUE_ACCESSOR,
      useExisting: forwardRef(() => CustomInputComponent),
      multi: true
    }
  ],
  template: '<input (input)="onInput($event)" />'
})
export class CustomInputComponent {
  onInput(event: Event) {
    // notify forms engine...
  }
}