You also associate the customer group with a platform user group. When a new customer account is created, a platform user is created as well and will be assigned to that group.
Benefits
You can use the platform´s permission system to set permissions for actions in the CMS and other extensions that support ACL.
Important note: In the Joomla configuration, you need to allow Site Login for any associated group at Site -> Configuration -> Permissions. See this page for details.