The question directed to Prometheus in line 5 relates to the message of the poem as follows; it raises the issue of suffering and its consequences and challenges the indifference of the gods.
The effect of the question
The question asks how Prometheus's compassion for humanity, which led him to defy the gods and steal fire, was rewarded. This sets the stage for the poem's exploration of the nature of suffering, the relationship between humanity and the divine, and the cost of defying authority for the sake of justice and compassion.
By implying that the gods might "despise" the sufferings of humanity, the question challenges their indifference and highlights the stark contrast between their divine realm and the harsh realities faced by mortals.