This first and favorite function is like the captain of a ship, having the most important role in guiding us, and it becomes the type core of our conscious personality. This mental function is called the dominant function. During the first part of your life, you come to rely on your favorite function, and you tend to develop the most skills with it. Some people, for example, give the most weight to their Intuition. They trust that function the most and they are the most energized when they use it.
Functions develop by being used consciously and purposefully for things that matter.
As the dominant is used, it becomes strengthened and differentiated from the other functions. We tend to have the most skills and conscious use of this function, and we tend to trust it the most. Extraverts by definition prefer to live in the outer world. Therefore they use their dominant function in the outer world. They put their best foot forward.