The stuffing is there to both make the box appear larger to the speaker and to stop internal resonances. The way it works is the air moving in a bare box moves in a straight line to the walls, in a stuffed box the air has to weave around the fibres, creating a longer path. In a sealed box (where the box should be loosly filled to the top) this can make the box appear up to 3 litres larger or more for big boxes, resulting in lower bass extension with minimal booming as the actual volume of air hasn't increased.
The fluff also absorbs the back wave, helping attenuate the reflected sound from coming out throuhh the cone, which results in cleaner sound. In a vented box, the result is the same, but you only line the walls with 1 to 2 inches of the stuff, and keep it away from the ports so the air can move freely.
Hope that helps...
XF died, Now It's a 6-cyl Cortina!
Rear ended 3 times, 4 close calls.
All while I was stationary, still counting the idiots..