There are several scenarios with respect to copies and updates: The advantages of SSOT architectures include easier prevention of mistaken inconsistencies (such as a duplicate value/copy somewhere being forgotten), and greatly simplified version control.
Common examples (i.e., example classes of implementation) are as follows: An acknowledged prerequisite (of the notion that any given single source of truth can exist) is that it depends on the ontologic condition that no more than a single truth (about any particular fact or idea) exists, an assertion that is ontologic in both the IT sense and the general sense of that word.
The broadest contexts (and thus thorniest, regarding ontologic discrepancies) require adequate epistemic regime comparison and reconciliation (or at least negotiation or transactional exchanges).
Typically the MDM acts as a hub for multiple systems, many of which could allow (be the source of truth for) updates to different aspects of information on a given entity.
One last advantage is that through this system the Shared Database pattern can be implemented, another technique not mentioned to obtain a single source of truth.
[4] In software design, the same schema, business logic and other components are often repeated in multiple different contexts, while each version refers to itself as "Source Code".