The primary function of any story's opening is to engage the audience.
While it may seem logical to begin with the details of the "main attraction", an appetizer is always appreciated. This short piece, seemingly unconnected to the prize character, sets the tone for further enjoyment.
During the telling of this seemingly unrelated incident, you bond with the storyteller. You become pulled into the rhythm of the experience. You start to anticipate receiving the remainder of the promised tale.
After you are securely engaged in the story, you can relax and enjoy what unfolds. You know that as wonderful as the "appetizer" was, the entree will be just as delicious. This is because it will have more depth, characters, sustenance. As you continue the account you come to anticipate more, to sate your curiosity.