With growing concerns over Data Confidentiality/Security and Privacy growing, administrators have to take enough care and opportunity where possible to ensure securiry and privacy is maintained. Security/Privacy concerns are both physical and electronic.
For instance, a passby individual can actually be looking at a confidential legal document which not everyone has access to or a PC left unattended by a user can prove to be a lethal weapon for a passer by.
Windows luckily helps protect against this passer by activity using simple screensaver functionality. Admins can Auto-lock user desktops or force log off user after a defined period of inactivity.