[SPDK] Clarification regarding VFIO driver bind in the setup.sh

I needed a clarification regarding the VFIO setup script for SPDK (https://github.com/spdk/spdk/blob/master/scripts/setup.sh). 
In the linux_bind_driver() method, we have the following lines:
	echo "$ven_dev_id" > "/sys/bus/pci/drivers/$driver_name/new_id" 2> /dev/null || true
	echo "$bdf" > "/sys/bus/pci/drivers/$driver_name/bind" 2> /dev/null || true

I'm wondering if both steps are necessary? It looks like if we set the vendor_id, device_id into "/sys/bus/pci/drivers/vfio-pci/new_id", this will automatically assign all the VFs (which are not assigned to the NVMe driver) to the VFIO driver. Hence, the second line (echoing the $bdf to the bind file) becomes redundant. Is this correct? Any reason why both of these steps are required? Thanks.