View the original community article here
Last tested: Sep 12, 2019
If "set roles from groups" is enabled, "LDAP/SAML users are given roles according to their "group to role mappings". Basically, you set a role in Looker to map to a LDAP/SAML group.
If "set roles from groups" is not enabled, then it gives the "default role" and "default group" as specified to ALL new users.
Reference: Looker-SAML setup guide, Looker-LDAP setup guide