Because they don't have much going on in the way of nutrition and they are very handy for eating on the go. Low fat, low protein, and actually not as many cals as you might think.
They are just kind of helpful to have handy because most of the prepackaged bar-type foods are hugely fortified with vitamins and so they are a better choice than let's say a power bar when you're in the car and need a quick snack.