Group policies are applied on users or computers based on the OU they are in. Just for the heck of it I will say that policies applied on user accounts are processed when a user logs into the machine and computer based policies are applied when a computer starts/reboots. Once GPO has been applied a lot of times there is a need to create an exception for it. We can use Group policy loopback settings to apply the user policy based on the computer user logs in.
For example, consider a standard group policy of screen saver turning on every 60 min when there is no activity on the user workstation. User might also have access to a particular server or workstation where the standard policy is not required and another policy of say 120 min screensaver time out is required due to user roles or business requirement. So if a group policy is applied on user accounts then how will it be processed differently when a user logs in to a different workstation/server?