"The Falling Girl" is told by an omniscient narrator. In omniscient narration, the narrator knows and shares feelings, thoughts, and actions of the characters. However, they are not usually one of the characters. Throughout this story, the omniscient narrator describes what is happening while Marta is falling.
To understand why the reader is told what Marta sees on the way down, it is important to note that the author included many elements of magical realism. Magical realism is a type of fiction that pairs real world elements with surreal elements and relies on vivid imagery to help tell the story. Magical realism is often used to make various points about reality and society while the author doesn't directly state their opinions or feelings in the story itself.
Also, "The Falling Girl" is told through the observations of both Marta and the onlookers. It is arguable that the reader is meant to "see" what everyone sees in a story where everyone is looking at everyone. It is also safe to assume that the reader isn't shown everything on the way down, but rather, we're shown the elements that move the story along.
As Marta falls, each description helps the reader understand where she is both in physical space and chronological time. Therefore, the visuals and interactions carry the entire plot and are necessary to decipher the story's meaning and momentum.