Geodatabases on the SQL Express platform were traditionally managed solely through ArcCatalog. This was done to provide an easy user experience to the geodatabase admin. Through ArcCatalog there were options for adding users, backing up and restoring the database. There were also options for viewing and managing user privileges. We provided what we felt was a few (suitable and easy to understand for SQL Express admins/users) roles that admins could use to administer privileges. These are the roles that you were asking about. We did not however, provide an option for creating new roles.
With enterprise geodatabases on SQL Server we do not limit what roles you want to use and do not make any assumptions about the roles you want to create. This gives more flexibility to admins who want more discreet or more general roles for their organization. These admins typically configure and manage their geodatabases using a combination of SQL Server Management Studio as well as ArcCatalog.