Omnivorous animals are faced with a dilemma, or a paradox [why eat spices]

"... Omnivorous animals are faced with a dilemma, or a paradox. One the one hand they are equipped to try new food sources and have a vast range of choices, which enhances their chance of survival; on the other, any strange food is potentially harmful and may cause discomfort, even death. The omnivore's capacity for eating nearly everything, then, is tempered by an innate distrust of novelty.
...
Culinary tradition is a guarantee of safety, and usually of nutritional wisdom: if it worked for your parents, it will work for you.
Cuisines are also given to characteristic flavorings, which are most often their most distinctive aspect, and, which have little, if any, nutritional significance... These flavorings are usually carefully maintained among immigrant groups whose normal staple foods may be unavailable in their new home, and suggests an analogy to the well-known habit of animals of 'marking' their territories with their own scents. Strong distinctive flavors mark our food as familiar and so acceptable, and may help ease the introduction of new staple foods into the cuisine. And, on the other hand, the shared preference for certain flavorings can help reinforce a sense of social solidarity, a feeling of community. The popular interest in exploring novel, exotic cuisines, so evident in the current variety of restaurants and cookbooks, is a development only of the least three or four decades. "

On the topic of why we eat spices (see Darwinian Gastronomy earlier) we can rely on Harold Mcgee's 'On food and Cooking' to provide the extremely sensible argument above. It's based on research done by psychologist Paul Rozin. It gives a complete new meaning to the notion of edible geographies. One word though: this is copied from my old edition of the book, not the updated one in which I have not been able to find it.