Final answer:
Apollo defends Orestes in his trial because he had commanded him to avenge his father's death, effectively involving himself in the act for which Orestes is on trial.
Step-by-step explanation:
Apollo defends Orestes after the latter is accused of the murder of his mother, Clytemnestra, to avenge the death of his father, Agamemnon. Apollo's defense is due to his own involvement in the case, as he had earlier commanded Orestes to take revenge on Clytemnestra, effectively making himself an accessory to the act. This responsibility compels Apollo to stand by Orestes during his trial at the Areopagus, arguing for his acquittal based on the divine commandment he had given.
In dramatic literature, the involvement of gods in human affairs often serves to illustrate themes related to justice, fate, and moral responsibility. Apollo's role highlights the conflict between divine and human justice, as well as the concept of pious duty—themes central to ancient Greek tragedy.