What is the purpose of Primary flag on Customer sites addresses?

Summary:

On Customer Sites, what is the Primary flag used for on Address Purposes..?

What i assume is that the site with Primary flag checked will be defaulted on the sales order/invoice as Bill-to/Ship-to address, but what we notice in the instance is that the Address marked as Primary is not being picked up as the default Ship-to address on the sales order.

So just wanted to understand what's the purpose of this Primary flag?

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