I don't have a propane smoker, but my guess with it is that it would allow more heat to escape. A charcoal or wood fired smoker would be different. I usually refer to the top damper as the "damper" and the bottom one as the "draft", same as a wood stove. If more air is flowing into and out of the firebox, you get a forge effect. A lot of heat at the coals, but maybe losing heat out of the top, so less in the smoker. Shutting down the top damper but leaving the bottom one open, will allow air to the fire, but hold in heat. You might get some of that effect with propane.