There's basically 4 things you gain, 3 of the automatically.
- More Power
- Flexibility in the direction of the flash
- Control. You can adjust the strength of the flash, so as to add only a touch of it.
- The ability to move off camera, with am optional external trigger (Most Nikon and some recent Canon cameras don't require an external trigger, see your camera's manual)
With built-in flash, all you can do is light it full on. This leads to bland pictures, without much interesting going on.
With the dedicated flash and no external hardware, you can actually point the flash in a different direction. A common thing to do is to point it straight up. This will "bounce" the flash on your subject, giving them a much more natural look. It will also tend to light up the whole scene much better, and almost never produces red-eye. It also avoids weird shadows, as the shadows tend to be in the downward direction.
These two shots, the fist one uses bounce flash, the second doesn't. Notice the second one has alot more reflections, is blown out in some areas, and generally has less detail. I think I used a diffuser for it, but it's still harsher then I would have liked, but sometimes you've just got a moment to get a shot, so...