Normal synchronization can take some time, during which the modified data would not be available on other servers. For example, suppose that in your setup you have different applications talking to the directory. You change your password on Server1. With normal synchronization, it is some time before this change is synchronized with Server2. Therefore, a user would still be able to authenticate to the directory through an application talking to Server2, using the old password.
Figure 10-1 Need for Priority Sync
In large deployments, when the critical data of an object is modified, changes need to be synchronized immediately. The Priority Sync process resolves this issue.