Actually it keeps a record of the _changes_ to the disk directory, which is quite a bit faster than actually changing the directory each time a file is added, edited, or deleted.

The consensus is that this is a good thing for disks that hold your OS but not as necessary with disks that hold only your media files. It's recommended that you turn Journaling OFF on your media drives.

A more detailed explanation can be found here: http://www.lafcpug.org/features_file_journaling.html