Automatic batteries normally aren't sealed. They have a hole that goes down into the actual battery and if
juice gets in there, your battery will be toast with no hope of recovery. For this reason, you cannot drip with an auto battery and leaking carts are always a major concern. Some auto batteries are also very sensitive so wind and loud noise can activate them, although this has been fixed in newer models.
Manual batteries are sealed, so they're perfect for dripping. Juice can't get into the actual battery and ruin it. They also give you more control over your
vaping experience, as Strigoi already said. Manual batteries will only ever be activated by pressing the button, so if you carry them in your pocket the button might be pressed accidentally. A lot of people just use the clips from pens to cover the button, but there are many sites that sell button protectors for cheap.