The subsidies are generally to have spare production capacity, so as to reduce the risk of famine that can occur from the capitalistic incentives of optimising the system for efficiency above resilience.

(Not that the subsidies are always actually the most sensibly set out: but the general idea of subsidizing farming is an important one)

> The subsidies are generally to have spare production capacity

Maybe originally, but not anymore. Exhibit A: See America's waistline and the reason behind it (hint: farm subsidies and SNAP, two sides of the same coin).