UNIX systems require an orderly shutdown procedure to cleanly terminate services and flush filesystem data.
Rebooting a UNIX system is usually only necessary to update the OS kernel or certain boot-time configuration parameters, or as a last resort to deal with certain kinds of extreme hosage.
In general there is no need to reboot to update service configurations; individual system services can be restarted individually.
On a multi-user system with interactive logins, it's polite to give some advance notice for shutdowns so users can finish their sessions cleanly.
Common UNIX shutdown/reboot commands: