The Sync Mode (previously known as Merge Mode) saves a great deal of time for the editors, and for cases when two or more translators are working simultaneously on the same file, or when one has to maintain translations for several branches of software.
Lokalize allows quick navigation through units that differ, and displays word-by-word differences. Also, Lokalize has two Sync views - Primary Sync and Secondary Sync. They are identical, but the former is usually used to merge translations and second to keep in sync translations for two software branches.
After you copied translation from auxiliary file (synchronized it), any subsequent changes made to this unit will be replicated back to auxiliary file.
One use of Sync Mode is reviewing changes made by (new) contributors, when you cannot be sure of the quality of the work done.
Open a base file, then drop its changed version into the Primary Sync view, followed by Alt+Down or Alt+Up (remember that shortcuts may be modified in a usual way for all KDE applications) to navigate through entries that are different.