Referencing an earlier question: If I fill the barrel with gravel, why do I need the barrel at all? Couldn't I just terminate the drain into a gravel pit?

Yes, that method will work as well, however the larger stones inside the barrel allow it to keep its shape while being burred, and create larger pockets for water to fill. This increases the volume of water that the well can support over a traditional gravel filled well. You can buy a dry well that does not need to be filled. These effectively double the volume of the dry well, but they are engineered to a greater degree than the plastic barrel pictured here. I'm a bit old school and I can't justify burying a $65 trash can in my yard, so I like to use gravel filled wells myself.