It allows the smoke to be circulated over, under and around the meat thus introducing a smoky flavor to the meat being barbecued. There should be at least 2 inches of clearance between the meat and the inside wall of the cooking chamber. Having such distance allows for even air flow and prevents excess radiant heat from the surface of the cooking chamber. The cooking chamber thermometer should be situated in the lid or attached to the chamber at a vertical location equal to the cooking grate. It should not be 6 plus inches above the cooking grate.
Heat rises, that's a given.
The variation can range in excess of 50 degrees. A key factor is that the cooking chamber should be large enough to handle all of the barbecuing necessary for a large party. You know better the qualifications. Unless you are using a small smoker, the cooking chamber generally has 2 cooking surfaces. It is here, in this lower portion of the cooking chamber, that most of the cooking is accomplished.