Fermentation is basically the opportunity to have a billion years of evolution do the heavy lifting for you in creating flavor. Yeast for instance has an incredible complexity in its metabolic pathways. Beer, for the most part, is just sweet barley water- the majority of taste differentiation is created by different yeast strains - and how they are treated. Stress will make them create different compounds. They create different compounds at low temperature than they do at high. It’s an incredibly complicated set of machinery cultivated over thousands of years to fabricate deliciousness for us.