Yes, as mentioned before, you can bring in stuff.
As to why someone might not want flank steak, any number of reasons including dietary restrictions, monetary restrictions, not wanting to stand in line when they can have lunch at hand, etc.
I used to bring my own in, and I remember that a wet cloth in a baggie and a towel were especially useful for before and after handling food.
Plus, it's FUN to plan a picnic and pull out all the goodies from the pick-a-nic basket and hear the oohs and ahhs from the neighbors.