One of the main reasons I think we need to eat fermented foods to be healthy is that their flavors correspond neatly to the flavors we like. Fermentation of fruits and other sweet foods changes sugars to acids, making the food taste sour and we like sour food. Fermentation of proteins produces glutamate, which produces an umami flavor and we like umami-flavored food. With many foods, their fermentation produces many microbial byproducts, giving the food a complex flavor and we like complex flavors.
The connection between fermentation and complex flavor is well-put in a Saveur article about fermented foods:
[As a child] I only knew Claussen and other vinegar-cured pickles, the kind you buy in jars off the supermarket shelf, and I liked them just fine. But when I finally tasted a real picklethe kind made the old-fashioned way, fermented with nothing more than salt, water, and timeI realized what I had been missing. A vinegary pickle plows through your palate with its tartness (often in a most pleasing way), but a live-cultured, salt-cured, fermented one tells a more multifaceted story. It is sour, to be sure, but it tastes of something more, something elusive: It’s the flavor of Middle Europe captured in one bite. When I started cooking for a living, I realized that the complexity I’d tasted in that pickle is the hallmark of well-made fermented foods, which include some of my very favorite things to eat and drink: not just pickles, but aged cheeses, tangy sourdough breads, blistering kimchis, tart yogurts, winy salamis, and of course, wine itself.