What purpose does coffee serve in a chocolate cake recipe? I have a recipe for a chocolate buttermilk cake.  It's not constructed like most cakes, but it's always turned out OK.  I've always wondered what does the coffee in the recipe do?  

Is the coffee just there as an additional flavor?  (The cake never tastes strongly of coffee.)  Can I use a cheap instant coffee, or will a higher quality coffee make a difference?  Does the acid in the coffee do something?  Does the temperature of the coffee really matter?

Here's the recipe:


  3 cups flour, 2 1/2 cups sugar, 1 1/2 tablespoons baking soda, 1/2 tsp salt, 1 cup unsweetened cocoa powder, 1 1/3 cups vegetable oil, 1 1/2 cups buttermilk, 3 eggs, 1 1/2 cups freshly brewed hot coffee, 1 tablespoon pure vanilla extract
  
  Slowly combine the flour, sugar, baking soda, salt and cocoa powder.  Blend (on medium speed) in the oil and buttermilk.  On low, mix in the eggs, one at a time.  Add the hot coffee and vanilla and mix on low speed.
  
  Divide into two 9 inch round cake pans and bake at 350&deg; for 30-35 minutes.


from Caprial's Desserts by Caprial Pence and Melissa Carey
it's probably there to add depth to the flavor, since chocolate and coffee complement each other so well. i've used the instant kind in similar recipes and it's just fine.