why merge backing file?

Hi:
  when i resize an instance which use qcow2 and backing file; why merge backing file?

        https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py:4694
        try:
            utils.execute('mv', inst_base, inst_base_resize)
            # if we are migrating the instance with shared storage then
            # create the directory. If it is a remote node the directory
            # has already been created
            if shared_storage:
                dest = None
                utils.execute('mkdir', '-p', inst_base)
            for info in disk_info:
                # assume inst_base == dirname(info['path'])
                img_path = info['path']
                fname = os.path.basename(img_path)
                from_path = os.path.join(inst_base_resize, fname)
                if info['type'] == 'qcow2' and info['backing_file']:
                    tmp_path = from_path + "_rbase"
                    # merge backing file ?????????????????????????????????????????
                    utils.execute('qemu-img', 'convert', '-f', 'qcow2',
                                  '-O', 'qcow2', from_path, tmp_path)

                    if shared_storage:
                        utils.execute('mv', tmp_path, img_path)
                    else:
                        libvirt_utils.copy_image(tmp_path, img_path, host=dest)
                        utils.execute('rm', '-f', tmp_path)

                else: # raw or qcow2 with no backing file
                    libvirt_utils.copy_image(from_path, img_path, host=dest)

or just "cp -r inst_base inst_base_resize"?

after the instanck's disk merged, will cause block migration error

thanks :)