This table stores prior IDs in a generic fashion for use by third parties and/or custom development for syncing people data to MP where finding a new Contact_ID value is required when a previously stored Contact_ID value is no longer in our system.
Records are created when the Combine Contacts Tool is used. Records might also be created by third party integration activity and/or custom development.
If you cannot find a previous Contact_ID you can search this table (available via REST API) and retrieve the new Contact_ID.