The first and most obvious use of lights is to allow us to see what is happening on stage. Flooding the stage with light can simply do this. If you are watching a graduation or an awards presentation, this may be adequate but it is not the case in theater.
Directional lighting becomes important in focusing attention on the critical area of the stage at any given time. If you are watching a musical and someone is singing a solo, the attention needs to stay with the singer. Often a spotlight will be used for this but dimming all lights but those surrounding the soloist could do it.
What is probably the most important use of lighting for stage is mood setting. If you are having a tragic moment, you don't want bright or colorful lights. The lighting would be somber and effects may include the look of the shadows of leaves coming through a window and onto a wall. Then it follows that this would not work well for a comedy where brighter lights and softer colors usually work better.
It is important that that the lights be set correctly to achieve the desired effect. The director tells the lighting designer where the scenes are and what they want accomplished with the lights. The designer then sets the lights. This usually involves physically climbing a ladder and adjusting where the light is aimed, and adding any gels or gobos needed to get the correct look. Gels are specially made colored transparent film and gobos are frames that produce different shapes or patterns on stage, such as leaves.
The director and the lighting designer work together to make sure the lighting is seamlessly incorporated into the production. As moods change constantly in a production, so does the lighting and it is important to make sure there are smooth transitions. The light operator has to make sure that a move from a dark and gloomy bedroom to a well-lit living room is done in a way that is not even noticed by the audience.
The combination of all of these elements helps to bring life to the play, just as the actor reading his lines. While a good performer enhances the production, so does good lighting. It is the overall effect that gives the performance a polished look.