Nearly every regional cuisine has a fermented ingredient at its core. The convergence is not coincidence: fermentation solved several problems at once long before anyone understood why.
Fermentation was primarily preservation
Before refrigeration, a harvest arriving all at once had to be made to last, and controlled microbial action was one of the few methods available anywhere.
Acid or alcohol produced by the process makes the food inhospitable to organisms that would spoil it, extending its useful life from days to months.
Vegetables, dairy, fish, soy and grain were all treated this way in different regions, producing pickles, cheeses, sauces and breads from the same underlying principle.
Salt and temperature were the control mechanisms
The technique depends on favouring the organisms you want over those you do not, and salt concentration and temperature were the two available levers.
Regional methods reflect local conditions closely, with hot climates using higher salt and cool ones relying on cellar temperatures and longer times.
These parameters were arrived at through generations of practice, which is why traditional methods are specific about quantities that seem arbitrary until they are varied.
The flavours produced cannot be made another way
Fermentation breaks proteins and starches into smaller compounds, producing savoury depth and complexity that no amount of cooking generates from the raw ingredient.
This is why fermented products so often function as the seasoning base of a cuisine rather than as a dish in themselves.
Fish sauces, soy sauces, aged cheeses, cured meats and fermented pastes all play that role, and removing them changes the character of the entire cooking tradition.
Availability shaped which substrate was used
Regions with dairy herds fermented milk, coastal regions fermented fish, and areas growing soy or cabbage fermented those, so the technique is universal and the ingredient is local.
The resulting products then became identity markers, closely associated with particular places and defended as such.
Many carry protected regional status precisely because the method, the local microorganisms and the place are difficult to separate from one another.
Travellers meet them at the extremes
The strongest fermented products are usually the ones a visiting palate finds difficult, and they are frequently the ones residents value most highly.
Approaching them the way locals do, in small amounts alongside other food rather than alone, makes far more sense of them than a first taste in isolation.
They are also the ingredients most worth learning about in a market, since asking about them tends to produce a longer and more interesting conversation than anything else on the stall.