The opening shows the object of the quest. The middle is a series of adventures on the way to reach that object. This gives much room for character development and also to show the coming change in the character. Think of the roller coaster headed for that final curve and home. The ups and downs of the character, some triumphs and some defeats leading toward the goal. There is action and reaction in this section of the quest plot.
I've always found this a fun part of the quest story to write.