Stage directions help to convey the characters' emotions. Actors use their voice and body language to communicate their characters' emotions onstage. When a reader is silently reading a play, they don't have the benefit of seeing the actors' performances. Stage directions help to indicate the characters' emotional state, so that the reader can better understand the characters' motivations and conflicts.
Stage directions help to create atmosphere. The setting and lighting of a play can help to create a particular atmosphere. Stage directions help to describe the setting and lighting, so that the reader can better understand the mood of the play.
Stage directions provide insights into the director's vision. Stage directions can also provide insights into the director's vision for the play. They can indicate the director's interpretation of the text, and the choices that they have made in terms of staging and characterization. This can help the reader to better understand the play and the director's intentions.
In short, stage directions are an essential part of a play script. They help readers to visualize the action of the play, understand the characters' emotions, create the atmosphere, and gain insights into the director's vision.