11 Hours Ago
I'm not sure, but the big barrel could act as an oven and maintain the heat more homogeneously around the small barrel. In a bonfire, the top of the small barrel wouldn't be as hot, with the flames swaying around and the fresh air coming viciously between them.
The wood eats a lot of energy to be degraded. Even "dry", there is always water in it to put out, and the big molecules are hard to be demolished. So the temp could not be as height as it should.
The gas produced is what burns on the wood, but the energy is freed all along the flames, with a big displacement of air. That's not good to focus on the work. More over, only the outside of a given flame is actually the fire, locally really hot admittedly, but inside the flame it's colder, deprived of oxygen with a lot of unburned gas (obviously, because that's it which burns on the outside when it finds the fresh air). The amount of oxidative and reductive areas is completely unpredictable in an open fire, so is the temp. You can either get some carbon deposits or burn your piece.
An other point, the flames and the smoke are always in the way of the worker. The charcoal burns much more cleaner, you can approach it a little more and you can see clearly what's going on.
Last point for the clean side, are the meat tasty when it's cooked on a burning bbq? hell no, it takes a bitter taste due to all the unburned gas and tar condensing on it.