When you add coffee to food, you add complexity to the flavor. Coffee complements sweet foods like chocolate cake or brownies, red meat like beef or lamb, and nuts. Coffee can be used as an ingredient in recipes, but a quick way to add the the complex, brightening flavor of coffee to foods is to sprinkle it on some coffee salt. You can buy coffee salt or espresso salt, or you can easily make it yourself for a lot less money.
Make it yourself, the recipe is here: http://www.mnn.com/food/recipes/stories/how-make-coffee-salt