Fermentation, an age-old technique used to preserve food, has a long and rich history in the human diet. From the vast variety of cultured dairy products (cheeses, yogurts, sour cream, buttermilk, kefir), to fermented fruit and vegetables (pickles, sauerkrauts), to alcohols and breads, fermentation, souring, and culturing are a fundamental part of our food preparation and preservation strategies.
When a food is fermented, microorganisms transform sugars into alcohols (grapes into wine) or lactic acids (when milk becomes cheese or cabbage becomes sauerkraut), which prevents spoilage. In the case of dairy, fermentation can increase its digestibility to the point that those who are lactose intolerant or even allergic to the proteins in milk can tolerate it. This is because most of the lactose and casein have been broken down already through the fermentation process.
As with sprouting, fermentation increases the nutritional profile of the food, adding vitamin content and beneficial bacteria that are crucial to maintaining a healthy digestive tract and immune system. The process also neutralizes any potent antinutrients present in a food. The best example of this is soy, which is loaded with antinutrients and highly indigestible until it has been fermented. Even soaking and sprouting aren't enough for soy. I've noted this already several times, but it's worth repeating -- avoid all soy unless it has been fermented into tempeh, miso, tamari, or natto, a traditional Japanese dish you'll find in some Japanese restaurants.