In the early days of Greek drama there was only one actor on stage, playing all the different parts. The introduction of the chorus, then, was largely practical. It provided the audience with a distraction while the sole actor went off stage to change or prepare for his next role.
In due course, however, the role of the chorus became more closely related to the structure and tone of the play. In providing a commentary on the action, for instance, the chorus would serve to establish a much closer connection between the audience and what was happening on stage. The chorus could also help prepare the audience for dramatic shifts. This would allow the playwright more effectively to control the overall mood of the play.
The position of the chorus in Greek drama was by no means static, and it developed gradually over time. In Prometheus Bound, for instance, Aeschylus allows the chorus to become part of the action on stage rather than simply commenting upon it. His theatrical innovation drew the ire of Aristotle, among others, who accused him of diminishing the chorus's stature.
Yet we must never forget that Greek theater was communal in nature. In allowing the chorus to become an intrinsic part of the action, Aeschylus was also enabling the audience to experience a deeper moral engagement with the play. In effect, the chorus was no longer confined to the stage; it also interacted with the audience.