substitution -- why?  

You are absolutely right. Even many North Americans find the concept of substituting one ingredient for another a bit odd. As I explain in the book's introduction, my approach to substituting is not to attempt to reproduce an exact replica of the original. That is folly. Butter is butter and margarine will never taste exactly like it. However, there are times when substituting is fun or an unexpected necessity; I like to think of it as experimenting rather than substituting. For instance, I often make pureed butternut squash soup, but it's fun to switch things up and use other varieties of squash or use pumpkin to make the soup. Thinking along these lines helps me to be more creative in the kitchen. Hoisin is indeed a signature flavor, but if you think of hoisin as Asian barbecue sauce, that allows you to push its traditional applications and try it on dishes that typically call for other sauces that may be similar, such as American barbecue sauce. Barbecued chicken, for instance, is delicious when made with hoisin sauce instead of American barbecue sauce. Similarly, the North American "buffalo sauce" (melted butter and hot chile sauce) often appears on deep-fried chicken wings in North America (known as Buffalo wings because it originated in Buffalo, New York); but other foods also taste great when "buffaloed." Buffalo sauce is terrific on baked potatoes, in beef burgers, and on grilled shrimp.

The hoisin sauce example is somewhat extreme and was cited as among the most difficult substitutes (and perhaps among the most ridiculous). But this example helps to explain that substituting is not simply a means of replicating the original. Rather, it is a matter of altering ingredients, equipment, and/or techniques to achieve the desired results in your food. In some cases, you may have just run out of an ingredient or don't have a piece of equipment and need an available alternative. In other cases, you may want to intentionally vary the flavor of a familiar preparation. Other times, you may be looking to boost the nutritive value of a dish or avoid an undesirable ingredient; or you may want to take a shortcut to save time; or just experiment. I organized the substitutions throughout the book under the headings "If You Don't Have It," "To Vary the Flavor," "For Better Health," and "To Save Time," so readers can quickly find the type of substitute they are looking for. And for those who prefer not to substitute, many items are listed not as substitutes but as different varieties of the same food, such as varieties of eggplant. This sort of information can be enormously helpful to the home cook who doesn't know the difference between globe eggplant, Japanese eggplant, Chinese eggplant, and Thai eggplant, for instance, and how each one will alter the finished dish.

lamington you are absolutely right it is a very signature flavor.But I think what David is getting at is that we may not have that particular ingredient on hand at the time of preparation so allowing individuals to be able to "improvise" so to speak so they are not left hung out to dry. I don't know if any of that made any sense did it?