How does \Magento\Payment\Observer\SalesOrderBeforeSaveObserver handle the «sales_order_save_before» event?

Context: How does an order saving work?

958164/app/code/Magento/Payment/Observer/SalesOrderBeforeSaveObserver.php#L13-L43

/**
 * Set forced canCreditmemo flag
 *
 * @param \Magento\Framework\Event\Observer $observer
 * @return $this
 */
public function execute(\Magento\Framework\Event\Observer $observer)
{
	/** @var \Magento\Sales\Model\Order $order */
	$order = $observer->getEvent()->getOrder();

	if ($order->getPayment()->getMethodInstance()->getCode() != 'free') {
		return $this;
	}

	if ($order->canUnhold()) {
		return $this;
	}

	if ($order->isCanceled() || $order->getState() == \Magento\Sales\Model\Order::STATE_CLOSED) {
		return $this;
	}
	/**
	 * Allow forced creditmemo just in case if it wasn't defined before
	 */
	if (!$order->hasForcedCanCreditmemo()) {
		$order->setForcedCanCreditmemo(true);
	}

	return $this;
}