Pacing
An author can control the pacing of the story by varying sentence structure and length. Short simple sentences can be read faster while longer compound complex sentence structures take the reader more time to process. In an action scene, an author may want to have shorter sentences and even some fragments to demonstrate the chaos. When the author is writing a more descriptive part of the story with a lot of imagery and figurative language usually the sentence structure is much more complex. It helps the reader slow down and visualize. Varying sentence structure and length can help the author create a mood which helps to create suspense and/or develop a theme.