Subsistence farmers generally do not do this. Remember, the goal is not to maximize profit, but to avoid family destruction through starvation. If you only farm one crop (the best' one) and you get too little rain or too much, or the temperature is wrong that crop fails and the family starves. But if you farm several different crops, that mitigates the risk of any particular crop failing due to climate conditions, or blight (for the Romans, the standard combination seems to have been a mix of wheat, barley and beans, often with grapes or olives besides; there might also be a small garden space. Orchards might double as grazing-space for a small herd of animals, like pigs). By switching up crops like this and farming a bit of everything, the family is less profitable (and less engaged with markets, more on that in a bit), but much safer because the climate conditions that cause one crop to fail may not impact the others. A good example is actually wheat and barley wheat is more nutritious and more valuable, but barley is more resistant to bad weather and dry-spells; if the rains don’t come, the wheat might be devastated, but the barley should make it and the family survives. On the flip side, if it rains too much, well the barley is likely to be on high-ground (because it likes the drier ground up there anyway) and so survives; that’d make for a hard year for the family, but a survivable one.
— Bread, How Did They Make It? Part I: Farmers!, Bret Devereaux