Why do I need to declare "defaultButton" ? (In a component form part of view stack)

Question

After doing some research and some help from FlexGuy in another thread, I realize that I need to make sure I have custom components initialized before accessing parts of the component.  Just recently I was thrown for a long time when I tried to first click on a form field that was part of a component in my view stack, I'd get:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at mx.managers::FocusManager/focusInHandler()[C:\autobuild\galaga\frameworks\projects\framew ork\src\mx\managers\FocusManager.as:601]
With the debugger it's occuring in the focusInHandler method of FocusManager.as: (apparently _defaultButton is null.)
// restore the default button to be the original one
                if (defButton && defButton != _defaultButton)
                    defButton.emphasized = false;
                    defButton = _defaultButton;
                   _defaultButton.emphasized = true;
This prompted me, on a whim, to delcare a defaultButton:
<mx:Form id="empForm" defaultButton="{submitButton}">
which refers to my submit button:
<mx:Button id="submitButton" click="submitEmployee()" label="{submitLabel}"/>
My question, is why is this necessary? In some examples I don't see this declared at all, but I seem to need it? (I'm calling creationPolicy="all" to make sure my components are initialized but I still seem to have to this defaultButton declared?)

Answer 1

It's a way of proving that you are able to purchase content from that country's store i.e. that you have a valid billing address in it
From Why can’t I select None when I edit my Apple ID payment information? - Apple Support :
If you changed your country or region
When you change the country or region of an existing Apple ID, you must provide a payment method and update your billing options. If you want to remove your payment method after you change the country or region, you can change your payment information to None.