Here's why:
* It builds tension: The rising action introduces conflicts and complications that escalate the story's central problem.
* It develops characters: We learn more about the characters' motivations, strengths, and weaknesses as they face challenges.
* It sets the stage for the climax: The rising action leads to the story's most intense moment, the climax.
While "rising action" is the most common term, you might also hear it referred to as:
* Complications
* Developing action
* The middle (though this is less specific)
Ultimately, the middle of a story serves to propel the narrative forward and create anticipation for the resolution.