When AEM deployment topology is multi-publishers or publish farms based and deployed websites have features like user’s registration then there occurs a need of synchronizing the user’s data on each publishers. This is also applicable when you create the user using https:///security/users.html OR https://useradmin options and want that user account to be replicated on different publishers available in publish farms.
This synchronization is generally required because once an individual user register that registration request may go randomly to any one of the publish instance of publish farms and during next login session that user request may go to another publisher in the publish farms. If we do not synchronize the user registration data on all publish instances, then on another publish instances that user would appear as a new user and again registration need to perform by that user thus this creates bad user experience.
Below diagram depicts how the user’s data synchronization happen on publish farms via author instance.