The help explains clearly what the Administartors & Registered Users roles are for, and who gets assigned them, how & when etc. But for Subscribers, it only explains the who, how & when, not what it does:

Default Security Roles

DotNetNuke includes three ( 3 ) default Security Roles.

Administrator: This role permits page full access to add, delete and edit all pages and modules. This role also gives full access to the Admin pages.

Registered Users: This role permits page and/or module access to registered users of your portal. This role requires users to be logged in to the site. All users are automatically added to the Registered Users security role, unless the Authorized check box is unchecked.

Subscribers: All Registered Users are automatically added to this role upon registration. ( i.e. this role is set as Auto Assignment)  Authenticated users can unsubscribe or re-subscribe to this role under Membership Services on the User Account module.  The Administrator can choose to delete this role, or change it's settings as required.

So please could someone explain what it does? Or maybe it does nothing, but could be used for something?

Thanks.