The shutdown -r command makes sure that the file contents on the OS file system are written properly to the disk before a reboot. The volboot file is created in the OS file system, and is used to bring up the vxconfigd daemon after the system reboot. If the machine reboots for any reason without proper shutdown, and the volboot file contents are not flushed to the disk, vxconfigd will not start after the system reboots.