Why do smokers 'pack' their cigarettes?

Everyone I know who smokes does this, pounding the pack on something hard or the palm of their hand until all the tobacco gets packed down towards the filter. Not one of them seems capable of telling me why they do it. The best answer I ever received was: “You just do.”

Is there any benefit to packing? Does it change the experience in some noticeable way? If not, why bother?

And what’s with that freaking ‘lucky’ cigarette in the front turned upside down?