When Krusader users utilize some comboBoxes in the Synchronizer dialog: they often see bad suggestions because there are obsolete entries, mistakes and so on, but normal Krusader users can't remove those entries. For example, web browsers have Shift+Del in order to remove the thwarting entries.
With a KrHistoryComboBox, Krusader users are able to delete wrong entries from history lists, and therefore they can remove remove those bad suggestions that they were seeing.