TTL (Through-the-Lens) makes shooting a breeze.
Point the camera at your subject, set the exposure values in-camera and the amount of light hitting your subject is dynamically determined by a pre-flash emitted by your TTL-enabled light before each shot.
Unfortunately, using TTL has notable drawbacks, as illustrated in the video above.
When shooting in a studio or under very controlled conditions, using manual flash settings will ensure consistent exposures throughout the session.
Alternately, when shooting under circumstances with quickly changing ambient light and/or subject-to-flash distances, the benefits of TTL will be very much appreciated. [Sean]