The smoke chamber is the transition between the damper and the first flue tile. The opening and width of the firebox is larger then that of the chimney flue. When we narrow the chimney, the bricks are corbeled to provide this transition. If the interior of the smoke chamber is not parged smooth, then the smoke will draft slower due to the resistance bricks. Creosote can also form in the brick holes and smoke can seep into the holes and enter portions of the home. Unparged smoke chambers can be corrected by applying high temperature refractory mortar to the unparged portion. IRC building codes require the smoke chamber to be parged smooth:
R 1001.8 Smoke Chambers. Smoke chamber walls shall be constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete. Total thickness on all four sides shall be 8 inches thick and the inside surface shall be parged smooth with refractory mortar.