In some recipes, you can taste the subtle and delicious flavor of vanilla that comes from pure vanilla extract. In others, you can't taste the vanilla, but it enhances the flavors of other ingredients. Without the vanilla, your chocolate cake wouldn't be as flavorful.
When considering extracts of any flavor, know that when you use pure extracts, you only need about half as much as you would of an imitation flavoring. Be sure to read your recipe carefully to see if it calls for EXTRACT (pure) or FLAVORING (artificial), and adjust your quantity accordingly.