What is the point of a sack lunch - if any at all?

I had no idea sack lunches could be eaten by kids or parents at work! Personally I always pack them when the family goes for an outing to a park or other public lot, that way if they get hungry they can just eat their sack lunches instead of having to go home immediately or having to spend a bunch of money to grill when there's a ton of leftovers in the house.