This depends to a great degree on the individual reader. In antiquity, the plot and background of the play would have been known to the viewers before they entered the theatre. Thus what they would learn is more a matter of what the play revealed about the nature of people in relationship to the gods and fate than a particular subject matter.
For a modern student, the play serves as an introduction to a chronologically distant culture and set of values and beliefs. It can serve to challenge certain individualistic assumptions students may have with its emphasis on the importance of the polis and the inevitability of fate. It also contributes knowledge of the historical background out of which much of western literature developed.