If you leave your car running with the doors unlocked, somebody can take off with it. If you leave your PC while it is still turned on and you're still logged in, somebody could sit down and at your desk and mess with it.
That seems obvious to me, but apparently it isn't that obvious to everybody. The technology researchers at Gartner felt the need to call out "the risks of insider attacks associated with employees leaving their PCs unattended with active sessions running."
According to Gartner, a significant number of unauthorised access events occur when someone sits down at another user's computer. The analysts suggest businesses use "timeouts" for all PCs to ensure that users are automatically logged out or that PCs are locked, to minimize the risk of insider attacks.
"Someone else must have sat at my PC" is a typical defense to accusations of improper online behavior, according to Gartner. This excuse won't fly if companies take measures, it said.