The "All Users" Security Role is a specially defined role with special attributes. This role should be enabled only in applications in which no unique security limitations will be defined.
If custom roles will be created to limit security within the application this role should be deleted.
When custom roles and the All Users role are both defined within the model custom roles will not function properly. For example users will be able to see entire dimension sets even if they have been limited within to a subset within one of the custom roles.