SSSD does not currently run automatically on any system, nor is it presently required for your system to operate. We are an optional component, usable when you want network identity and authentication. Otherwise (and by default) we do not run on the system.
At this time, SSSD doesn't interact with local users at all. However, in the future we're planning on adding additional features for the desktop. At that time, we may start handling local user accounts as well.
(Such features include central or local storage of additional user data such as language preference, face browser image, etc.)