They are okay in stillwater but used more on a river with moveing water.
If you can imagine throwing a handfull of bait such as maggotts in a river, they would very soon end up way past where you are fishing, (usually quivertipping).
What you do is to load the baitdropper with bait, and make sure it is closed properly, then you lower it into the river at the spot you want your hookbait to be or just before it. When the plunger on the baitdropper touches the river bed, it automatically opens and empties the bait on the bottom where the flow is much slower, and will satay around that area for longer.
I wa sactually under arm swinging mine the other week about eight yds.
In short it is a method of getting your bait down to the bottom in a fast flow.