Music can have a number of different roles in a play. In terms of musical theater, such as opera, ballet, and musicals, music has the function of communicating the emotional heart of the action and the feelings, wishes, and thoughts of the characters. In musical theater, music involves a certain degree of suspension of disbelief on the part of the audience. The characters sing to each other or directly to the audience as if they are not even aware that they are singing. As the audience watches the characters on stage break into song, they are brought into the inner workings of the character and experience them on a deeper level. It helps us understand the personality of the characters, establishes a backstory, and moves the action forward. In non-musicals, a monologue can often achieve this same goal, but usually with less spectacle.

In non-musical theater, music can still play a role. Sometimes it used to help establish a certain mood or atmosphere. As in film, plays can use music to associate the action on stage with a certain theme. Certain characters and story elements will be associated with a particular musical theme to clue in the audience that they should be thinking or feeling a certain way. This is often done subtly when it is done most effectively.

There are, of course, plays without music. Even in these plays, however, there is sound, primarily the sound of the words. Where music is present, it is important for much the same reasons as the words are important.

In the first place, it may convey meaning. Music cannot convey thoughts as clearly as words can, but it may convey shades of emotion with much greater subtlety. Related to this is the creation of atmosphere, which may contradict the words and alert the audience to some incongruity (for instance, if a character is making a comforting speech, while the music conveys a sinister feeling of unease). Music can also link different scenes or sections of a play which contain similar themes and ideas by repeating the same musical pattern, like the leitmotifs in Wagner's music dramas.

Music, like language, also has an aesthetic purpose. The music may add beauty, grandeur, and pathos to a scene. It may also complement and heighten the effect of the words.

While it is difficult to grasp the exact theoretical reason that music is important in theater, everyone seems to agree that it is. Aristotle himself maintained that music was one of the six key elements of drama.

Source Music is music that would actually be occurring in the world of the play, such as fanfare upon entering a royal court. This is important for obvious reasons, as it helps to draw us into the world in the same way that a costume would.

Underscoring is the atmospheric music that would not occur in the world of the play and exists strictly to enhance or depress the mood of the action on stage. Underscoring is important because...