We're not schooled or experienced in the ways of the common thief, but it would seem to us that if you took the time, effort, and risk to don ski-masks and jack pizza from a delivery man, you might as well take his cash while you're at it. Seems like one of those killing two birds with one stone situations. That way you could buy a pizza the next time you get hungry.
Even if you're main target was the food, even if your hunger pangs were so dire and persistent that you could think of nothing else but a piping hot slice of pie, you're already committing a crime, so why not get a little extra dough, too? Perhaps the fact that we don't understand means we should stick to the writing thing and leave crime to the criminals.