Prior to the introduction of file notifications in MOVEit Central 3.1, in order for a task to notice a file, the task had to be scheduled to run repeatedly. Each time the task was run, it would check for files matching a given mask. Oftentimes, no such files had arrived, so no action was taken, and the task would be run again later by the scheduler. In order for a task to see a file shortly after its arrival, the task had to be scheduled to run often, causing extra overhead for both the remote server and MOVEit Central itself.
By contrast, file notifications allow you to run a task only when files it's interested in are actually available. And with file notifications, the task runs almost immediately after the arrival of a file.
The MOVEit Central component that implements file notifications is called the "notifier".