Farm subsidies are still important, they are what stabilizes our food supply so we don't pay 5x as much for bread when we have a bad year for wheat. Its main purpose is more like food supply insurance, if you only produce EXACTLY what you need/get paid for, then even say a 5% less yield in a large area than expected, maybe from weather, storm, or other environmental conditions, could result in food shortages and huge price spikes. You always plant extra to keep you from starving, but if it was completely unregulated, nobody would ever want to overproduce what they could sell or it is pure lost profits. If you couldn't sell 5% of grain last year, you plant 5% less this year, but if this year was a bad season for other farmers, now you don't have enough to meet demand and the prices skyrocket, good for you personally but bad for thousands of other people that need to eat.
Farming is already not perfectly predictable yield to yield, when you add in even wilder price swings it only crushes farms under further financial risks and leads potential famine.