This is to allow customers who have Webex users in external domains (such as gmail.com, outlook.com, yahoo.com) to enable SSO in their organization while still allowing the external users to use a Webex authentication flow. The "partial" SSO authentication flow would be required only for internal users with mail ID domain owned by the customer and claimed in Webex Control Hub.
Currently SSO toggle is an Organization-wide setting redirecting all Webex users to configured Identity Provider. In a scenario where Webex users include domains not owner by the customers, they cannot be added to IdP directory (e.g. Azure AD) and authenticate.