Using GP to enforce a default setting - Good or bad?

I have seen many organizations enable a policy and set it to the normal default value. As I understand, every policy you enable takes up processing time when GP is applied. Why would this be done and what is the value of using GP to enforce a setting that would normally default?

Example: "By default, file downloads are disabled for the Restricted sites zone. However, we recommend using Group Policy to enforce this restriction of the Allow File Downloads setting to ensure that users cannot download files when they are in the Restricted sites zone."

Thanks! 

Charlie Newman