If you have the same contact on multiple lists, you want to be able to access all that person's information in one place, without having to go from list to list to see the details. This feature synchronizes a single contact across all the lists which that contact is on. You can choose how your contacts are updated based on new uploaded information. This allows for more integrated and specific reporting information, which is useful for targeting. It also helps to provide cleaner list management and allows data to be easily updated so your contact lists stay current.