Like more modern choruses, the Greek chorus would sing songs concerning the drama and characters in the play.
Greek choruses served to provide the viewers with plot exposition, essentially acting as narrators for many parts of the drama.
The Greek chorus often acted as audience surrogates, questioning the other characters' motives or warning them about the consequences of their actions.
On a more practical level, the songs and speeches of the Greek chorus gave the other actors time to take a break while allowing the scenery to be adjusted and other changes made to the set.
In the days of Greek theater, sets were very sparse and special effects nonexistent. The chorus lent a sense of epic to the setting, granting the play a grandeur it might not otherwise possess.