Fermentation preserves food and predigests food, making it easier for us humans to digest. I eat a lot of fermented foods, which has helped with my digestive issues.
Fermentation is a food preparation technique that humans have been doing for thousands of years. Fermenting is when microbes transform a food, so the final product is alive – full of good bacteria – and often sour. This is not the same process as canning or adding vinegar to vegetables.
Picture a rural farmer in the 1800s who gets a big harvest of vegetables all at once, but does not have the means to eat all of the veggies right away. The farmer does not have a refrigerator, so he or she needs a way to keep the vegetables fresh, or else they will rot and be sent to the compost pile. Fermenting the vegetables in salt and water kept them edible for potentially months after harvest and prevented the veggies from rotting.
Most, if not all, traditional cultures, had (and still have) at least one commonly fermented food. Eastern Europeans have sauerkraut, Koreans have kimchi, French have cheese, Mexicans have tepache, etc.