It is a requirement when you have updated certain packages especially kernel.
reboot
If you have only updated userspace (graphics, desktop etal.), userspace can be restarted using
systemctl soft-reboot
The Add & Remove Software application (if used to update) will prompt when restart is required and offer to initiate the restart.
Or you can check using the commandline (before executing the sync) using a small utility script
See → [root tip] [Utility Script] Check if updates may require system restart