Why do people who smoke smack the cigarette box before they open it?

Because they want to call attention to how "cool" they are.

There is a myth that by slapping the box you can compress the tobacco to get a more even burn. This isn't true though and you often end up loosing a lot of tobacco from each cigarette.

A cigarette is meant to be smoked how they are sold so don't buy into any of that crap about having to spank the box to get a better smoking experience. Anyone who tells you otherwise is probably still in high school and is just imitating what they see on TV.