It is not always a critical security update that requires a restart when installing Microsoft updates. The reason for the restart is that some updates require access to files currently in use. In order to avoid a conflict when installing the update, the a prompt is provided for a restart. The installation can then be made without conflicts.
Based on the above explanation, when the update fixes a security-related issue, your computer is not safe until you reboot and the update is installed.
The Microsoft Update Product Team provides a complete explanation of Automatic Rebooting, Update Deadlines, and Policies to Change Automatic Rebooting Behavior in Windows Update and Automatic Reboots.