Functions of Lighting
Stage Lighting has multiple functions, including:
Selective Visibility: The simple ability to see what is occurring on stage. Any lighting design will be ineffective if the viewers cannot see the characters, unless this is the explicit intent.
Revelation of form: Altering the perception of shapes onstage, particularly three-dimensional stage elements.
Focus: Directing the audience's attention to an area of the stage or distracting them from another.
Mood: Setting the tone of a scene. Harsh red light has a totally different effect from soft lavender light.
Location and time of day: Establishing or altering position in time and space. Blues can suggest night time while orange and red can suggest a sunrise or sunset. Use of mechanical filters ("gobos") to project sky scenes, the moon, etc.
Projection/stage elements: Lighting may be used to project scenery or to act as scenery onstage.
Plot(script): A lighting event may trigger or advance the action onstage.
Composition: Lighting may be used to show only the areas of the stage which the designer wants the audience to see, and to "paint a picture". 
While Lighting Design is an art form, and thus no one way is the only way, there is a modern movement that simply states that the Lighting Design helps to create the environment in which the action takes place while supporting the style of the piece. "Mood" is arguable while the environment is essential. 
- McCandless, Stanley (1958). A Method of Lighting the Stage, Fourth Edition. New York: Theatre Arts Books. pp. 9–10. ISBN 978-0-87830-082-2.
- Gillette, J. Michael (2003). Designing With Light: An Introduction to Stage Lighting, Fourth Edition. McGraw Hill. pp. 9–10. ISBN 0-7674-2733-5.
- Gillette, J. Michael (2003). Designing With Light: An Introduction to Stage Lighting, Fourth Edition. McGraw Hill. p. 10. ISBN 0-7674-2733-5.