Its like a little tank that hangs on the back or inside of your tank, its used for growing food,algae, or containg specimens that wouldnt fair to well in your main taknk or a fish that needs to be isolated.
The obvious benefits are a greater water volume. ANother is its like a mini filter in some ways if you have macroalgae or mangrove plants.
Most people though just use it as a place to culture copepods,amphipods,mysis shrimp and other types of food.
I'm tired of getting pointless messages. If your going to start a conversation or ask me something, keep up with it.