We are lucky to live in a age of a dramatic global decline in famine, but a large scale war or natural disaster could potentially unravel this progress in a devastating way.
Subsidies can do preventative tasks that insurance cannot -- like securing additional capacity of food stores and farm land that farmers wouldn't normally do unless they were being paid to do so.
I'm no agriculture nerd but isn't much of the US particularly well suited to growing corn, regardless of subsidies?