Fermenting the milk reduces the lactose sugar, which some people have an intolerance to.  It also produces beneficial acids in the process that help to kill pathogens and improve the absorption of nutrients.  Cultured products also contain the beneficial bacteria that help control pathogens through the formation of peroxides and bacteriocides.  These bacteria also produce several vitamins and serotonin for the body.  Many of these bacteria also provide some proteolytic activity helping to break the proteins in the milk down in to smaller units making fermented dairy products easier to digest.