Why must override function: writeToParcel from interface Parcelable in android?

Question

I have a class: class PartStoreModel implements Parcelable which contains some properties such as: id, name, address. In this function I must override function: public void writeToParcel(Parcel dest, int flags) (I left this function empty, don't do anything):

@Override
public void writeToParcel(Parcel dest, int flags) {
// Do nothing here
}

// This is how I save on configuration change:
@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putParcelableArrayList(KEY_INVENTORY_GROUP_SAVED,
            mPartStoreModels);
}

The problem is that It worked perfect!. So why I have to override writeToParcel function, when I can leave it blank? Can anyone explains it? (I usually override writeToParcel function, and PartStoreModel(Parcel in) as write and read function, but nothing is better than leave them blank!)