Regular surpluses can cause famines. This is what happened in East Africa in the 1980s. Cheap grains from elsewhere (Europe, US) caused farming to become unprofitable. Domestic/regional traditional farming of grains largely ceased as farmers moved to the cities. This happened very quickly, so consolidation and mechanization of farming to become competitive never happened. When cheap imported grains became unavailable in the 80s, for various reasons, it was too late. (The war in Ethiopia is often cited as the immediate cause, but people have always managed to farm through wars, usually at least enough to avoid the Ethiopian situation.)

It's an extreme case, but that same sort of pattern has happened repeatedly throughout history. Keeping some amount of farming economically sustainable is important. You don't necessarily need direct public subsidies, but you definitely want to avoid long periods where prices are too cheap to make farming of important crops not economically viable.