| One word: business.
And, of course, a few hundred more in explanation. Picture, if you will, in your mind's eye, a man standing in the street, or listening to the boss, or having a conversation in his parlor. For the most part, he's probably standing still. Yet his subtle body language- eye movements, facial tics, small shifts of weight, tells us that he's actually active, aware of and involved in the world around him.
Now, picture another man on stage or on film, acting out those same tasks. He's probably at least several feet from the camera or audience, in unnatural makeup and lighting, potentially wearing a difficult costume, trying all the while (perhaps unsuccessfully) to not only pretend to be a different man, but to, as that man, perform a specific course of action and speech, with little room for deviation. Chances are, you can't pick up on (or he isn't even expressing) all those subtle hints to the extent you could with the first man. If he's standing still, he looks dull, zombielike, removed from the situation.
For this reason, actors, directors, and writers often try to give the characters something minor to do with themselves when they're not engaged in any other sort of action, something to signify that yes, they are real living people. This is called "business". But what can you use as business? You could have the actors carry or hold objects, but that might seem odd and would need to be worked into the story. You could establish some physical habit or behaviors for the characters - flipping a coin, adjusting their glasses, or looking around, but you run the risk of making them seem nervous, crazy, or just plain unnatural. What you really need is to find some minor physical activity which doesn't interfere too much with the actor's job of speaking, acting, and progressing the story, yet which appears to the audience as a perfectly unremarkable thing for a normal person to do.
You need something like smoking - a common habit, mostly unobtrusive and believable, which ties up the actor's hands for a while, coming complete with a set of widely-recognized (and sexy) ritualistic movements and behaviors. I know that all the anti-smoking activists would like you to believe differently but the prevalence of cigarettes in movies isn't just some evil tobacco company plot.
But anyway, this node's not about smoking, it's about eating. And with good reason. Smoking may be common, but it's not completely innocuous. Smoking carries overtones and implications that you may not want associated with the character, especially if you have him or her smoking every time there's a lull in the action. So what else can we use? Of course, eating! Everyone eats, and there are few situations in which it would seem outright strange to have food available. The actual consumption of food may interfere with dialogue or acting, but as StrawberryFrog notes, it's easy to gloss over that part altogether. Eating is one of the few behaviors imbued with more meaning and ritual than smoking (and potentially sexier ones at that), and social conditioning has brought us to associate them with increased sociability, conversation, and drama. People talking over dinner or while sharing a street vendor's fare seems natural, and in a dramatic work in which multiple days or even years are covered in the space of a few hours, it is completely believable that a significant proportion of the interaction between characters, and thus the scenes portrayed, takes place while eating.
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