Null customer_address_id on sales_flat_order_address table

Question

Does somebody know why field customer_address_id is null on sales_flat_order_address table? As regards to order for customer who has an address.

In class: Mage_Sales_Model_Order_Address I found this:

protected function _beforeSave()

{
    parent::_beforeSave();

    if (!$this->getParentId() && $this->getOrder()) {
        $this->setParentId($this->getOrder()->getId());
    }

    // Init customer address id if customer address is assigned
    if ($this->getCustomerAddress()) {
        $this->setCustomerAddressId($this->getCustomerAddress()->getId());
    }

    return $this;
}