Many citrus recipes will often tell you to use lemon, orange or lime zest instead of the juice. But why is this? It's because the zest has a very high concentration of essential oils which provide a floral flavour and aroma when exposed to heat.
Juice is quite acidic and often changes the balance of a recipe. Acid reacts with baking powder so it could affect the rise of a cake. Juice is also liquid and will make the batter thinner, affecting the cooking time.
But don't think that you should never use juice in a recipe. One of my favourite recipes, lemon meringue pie, contains lots of zest AND juice. If you want something to have quite a tart taste to it, then juice will provide that flavour.