Use synchronization to set up a new replication target by populating empty tables before activating replication. In this case, once the tables have been synchronized they probably never need another synchronization because changes in the source table are continuously replicated to the target.
A second use scenario is that you normally keep online replication turned off and synchronize changes from the source table to the target table at periodic intervals. In this case, you are more likely to want the “sum-total” changes rather than all intermediate modifications.