Best Practices with Restarting Windows Servers

If you patch on a regular basis, then you are rebooting at least once a month.
Reboot is needed for some operating system updates (It depends whether you use WSUS or similar tool, or there is simple update.)
The same as in 1, but for applications
Testing and troubleshooting boot
You can schedule a reboot when you are sure, that there are no important tasks like a backup, no automated import/export operations running, or when you expect that users are in the system.
Avoid unnecessary reboots and if you really need reboot, take into account the possible problem you should solve after reboot with errors.