Why should I use Validators.compose()?

When we create new FormControl/FormGroup/FormArray(AbstractControl) – coerceToValidator is called.

function coerceToValidator(
    validatorOrOpts?: ValidatorFn | ValidatorFn[] | AbstractControlOptions | null): ValidatorFn|
    null {
  const validator =
      (isOptionsObj(validatorOrOpts) ? (validatorOrOpts as AbstractControlOptions).validators :
                                       validatorOrOpts) as ValidatorFn |
      ValidatorFn[] | null;

  return Array.isArray(validator) ? composeValidators(validator) : validator || null;
}

export function composeValidators(validators: Array<Validator|Function>): ValidatorFn|null {
  return validators != null ? Validators.compose(validators.map(normalizeValidator)) : null;
}

So there is no need to compose validators before we pass it to an AbstractControl.

6/13/16 was added feat(forms): compose validator fns automatically if arrays from now on, Validators.compose is there for backward compatibility.